To take a totally different perspective on this than we have been addressing already...and rather putting the cart before the horse (I mean, we should get the auditions out of the way first and see if people get through!).
One of the more subtle 'dangers' of dealing with a show like this is, I think, that everyone tends to looks towards the end without worrying about the process that is used to meet that end. I am not greatly offended by the idea that, say, a Lee Mead can become a shoe - in for West End runs because he has been selected by a TV audience for the honour (though I am not a MT performer, so it conceivably bugs me less). In essence, though, I think there is even a kind of commendable quality in the fact that one of our own (or, if you prefer, several of our own i.e. the runners up) - frequently jobbing pros who are trawling the industry circuit looking for a break - are allowed a break by dint of such a show. Certainly, when the West End remains locked in a permanent 'attract bums on seats' stranglehold which dictates that all major parts go, these days, to anyone from boy band stars to alternative comedians on the basis that they are 'famous' and will bring in the punters, but when the employment of genuine, trained MT performers is as rare as hens' teeth, to have a genuine trained performer (however selected) actually taking the lead in a West End show seems like a breath of fresh air.
I don't doubt that anyone who has been well trained and knows their profession should be as capable in front of the studio audience as any, and as Alan says, why not go for it?
My issues with the format are rather different. Essentially, I think anyone who volunteers to sign up for a 'reality TV show' (however it's packaged) needs to be very careful at all times that their dignity, self - esteem etc. are protected. The trouble is, all TV needs to tell stories. We all complain about how there is no decent quality drama on TV these days; it's all reality TV. But however much money the TV companies save on costumes, they all still realise that drama is essentially what makes people tune in to a show. And even in reality TV land, they have to manufacture drama.
Partly, this is done by selecting only the people who are likely to behave most interestingly on camera - so you might say that the 'audition' process is flawed from the start; in a standard situation, Lord Lloyd-Webber (or whoever) wouldn't be under pressure to select a person who could sing, dance…and have a wobbler every time someone else did better than him in a competition, say. Alright, give shows like this the benefit of the doubt - and assume that it is genuinely only the best prospects who get through. What happens next?
Basically, the story gets manipulated. What the papers want, what the public want to invest in is: the story of the poor, downtrodden no hoper who is given a chance at stardom - their last chance to better themselves, or the story of the arrogant bitch whom everybody loves to hate, or the story of the kid who really hasn't got a snowball's chance in hell, because he speaks with a northern accent, but is kept in because he's funny…or whatever. Now, it's probably the case that none of these people are actually really any of these things they are presented as being - a tragic hero, a poison bitch, a clown - but the drama will work better if they are shown to be such. And, believe me, if you are lucky enough to get through auditions, then it is not the live shows that you should be worrying about (at least, no more, as a seasoned performer, than you should worry about performing on any stage) - it's the fact that you will be stuck living in a small house with 10 strangers for weeks on end, under intense pressure to perform, with a camera crew filming you practically 24-7. And they will use whatever footage they feel like (judiciously edited, of course) to tell whatever story is going to work for them. Probably everyone is likely to cry in a high trauma situation at some point - they can make it look like you are the sort of person who collapses into a blubbering wreck at the first sight of difficulty. Everyone will lose their temper at some point in such crowded, high stress conditions - they can make you look like a raging maniac if they so wish. You can end up being shown to be sycophantic, or superficial, or sex - mad or whatever, and while it will all have a small grain of truth (the camera can't record what isn't there) over weeks and weeks, all the foibles and intricacies of your personality will break through, and the public stand the chance of become accustomed to a constructed 'you' that is nothing like the reality.
Lee in Joseph is an interesting case, because he was always made out to be the strong, smooth, generally unruffled, ruggedly professional one of the troupe, and, lo and behold, he won. I felt sorry (the whole point, I suppose) on the other hand, for Lewis, for example, who seemed to me like a talented guy, but got told so repeatedly that he was whiny, and weak, and went to pieces every five minutes (I've no idea if that was the whole truth - I think it unlikely) that he lost most of his confidence, and ended up believing what they said himself. Or for Rob, who really looked like the ordinary working bloke he was, and, frankly, was never likely to have landed a job in the West End being a musical leading man, and would no doubt have been turned away at the very first audition as unsuitable for the role in the real world (which is not to say that he wouldn't have been great at some part more suited to his look - Bill Sikes, maybe, or Nicely - Nicely in 'Guys and Dolls'), but was kept on for the sake of evoking the story that even 'ordinary' folk have a chance to become stars overnight. They never stopped referring to him as 'Rob the Builder', of course, just to underline this amazing fact - that they had discovered a man who could do an ordinary job, and also sing nicely, the implication always being that this was a bit like discovering a horse that could also do interior decorating - you couldn't see what use you'd have for it, but boy, it surprised you.
So, what I'm trying to say, then, is you've got to be careful when you sign up to these types of shows - you must be prepared for what they are going to force you to live through for three months in order to be in with winning the 'chance of a lifetime'. None of this comes cheap, and I suspect for some of those who don't win, the problem of having so much of their dirty laundry aired on screen so that everyone has something to discuss over the breakfast table the following morning doesn't do them any favours in the long run.
Never forget you are a trained professional with talent, and you deserve to be respected as such; they are lucky to have found *you.*
That's my opinion, and I'm sticking to it.