Again, this comes down to a question of what is defined as 'amateur'. Many actors CVs carry student film credits when they are at a point in their career when they haven't done a great deal of more 'professional' screen work - this is because a) often, the schools are themselves well respected in the industry, and employ tutors and practitioners whose work is recognised as high grade and b) the students are assumed to be working towards active careers in the industry. The execution of *some* student films are a disappointment, but many are extremely well made, sometimes DOPed by the tutors, who are invariably industry professionals (at least at film schools per se, as opposed to universities, where they may simply be academics specialising in film studies) and make great showreel additions. They are certainly no more 'questionable' than an actors' own drama school credits - which you certainly ditch from the CV the moment you start working in better productions, but have a similar sort of validity - they are created by professionals (the tutors) working in tandem with the students.
Lines are increasingly hard to draw in this day and age, anyhow - many professionally mounted productions don't pay the actors much, if anything - which is a different argument entirely, but no one would deny the quality of the production. I recently saw a number of friends perform in 'Christmas Carol' at the King's Head, for instance - a wonderful production, which garnered rave reviews, and will look great on their CV's for many months to come - and yet, it was still profit share at the end of the day! As previous threads have pointed out (wisely), profit share productions may be dubious in all sorts of ways, but from the point of view of *intent*, they hope to make money and garner industry recognition for the performers and the company - in that sense, they're professional. A low - budget film that is seeking distribution is 'professional' etc. etc.
'Pure' amateur work is the type of material that is solely put on within the confines of a very limited forum - it is not intended to be competing with the professional productions of the capital or elsewhere - the casts are primarily made up of dilettante enthusiasts for acting, rather than those who consider acting a full time profession. And there *is* a world of difference between an actor who has to make ends meet by temping/serving bar/stacking shelves etc. in addition to performing, and a teacher/estate agent/retired colonel/accountant whatever, who does a bit of acting 'on the side' in the local village hall. This type of work is pointless on a CV - it can immediately be acknowledged for what it is - and Blake is right, casting directors will take you less seriously if you are seen to be taking *it* seriously. Rather than thinking you are an inexperienced neophyte actor who may need to be given a chance because you have, as yet, gained few decent credits, but are clearly working hard to try to, they will think you are a self-important prima donna who cannot tell the difference between actual work, and impressing your friends and family!
This isn't to say Reuben is wrong - you will probably find that 75% of actors (including myself) started off in amateur productions - only a few are lucky enough to immediately fall into being given a supporting role etc. in a professional production off the bat - and from then on, they are bitten by the bug.
But that's not to say quoting them on a CV is a good idea - there are a very few exceptions - somewhere like Questors has such a good industry rep that having performed there is worth mentioning, if you need to pad the CV out. Although I never did a great deal of amateur theatre, I did loads of good roles with university drama societies (and even got selected for the National Student Drama Festival on the basis of this)...but I have never listed them on any CV, other than mentioning them in my drama school application, because I realise they carry no weight within the industry whatsoever. There are many reasons for this, although one obvious one is this: in university groups/amateur setups, you can easily be cast in a part which would never be given to you in the actual industry in a million years, and it has been given because there is a very limited number of actors to cast from within such a grouping. This sort of thing is just not worth referencing.
If you feel that this experience *is* important to you, and reveals an extra side to your performance history, I would suggest adding a note or rider to the CV, under e.g. 'other information' and pointing out that you have experience of working with such and such an amateur theatre, particularly as this is often a long-term relationship, and have played such - and - such roles in your time with them. Then at least people can acknowledge it without being bothered by it. But even then, it may ring hollow, unless you really are just starting out.