There is definitely a logic in using IMDB Pro to bolster your pursuance of a film career if you are serious about aiming to have one (personally, I have blown hot and cold on whether I subscribe or not - and sometimes have, sometimes haven't, depending on resources and mood) - having the ability to ensure your profile accurately reflects what you offer the industry, posting reels of your work up, and being able, as Claire suggests, to reference and check valuable information is all important grist to the mill in your continuing efforts to source work.
BUT it should never be forgotten that IMDB is NOT an instrument designed to secure actors work in the first instance; it is a glorified fan site that has evolved into a directory of film, with a reputation for convincing casting directors (or, at least, casting directors who are this way inclined) as to which actors they should invest in, given the relative ranking (and hence, perceived saleability) of the actors concerned, and a reputation amongst ordinary fan boys and girls for being as error riddled as Wikipedia and about as trustworthy.
What IMDB *does* have is a virtual monopoly upon allowing actors access to *any* information on the pre and post production stages of projects that otherwise shun outside publicity. Whether you can use this information to personal advantage through application to the casting offices concerned at the appropriate moment seems to me as moot as whether you can do it in any other capacity; and the majority of the time, the same old adage must apply: unsolicited applications are rarely taken notice of by a production office if they are not specifically looking to cast to type at that precise moment.
Beyond that, as ever, how useful access to such 'second order' material proves really comes down, as so often, to how effectively you feel your career is being marshalled in the first place. If your showreel is in tip-top shape, reproduces well and is worthy of being displayed on site amongst a host of top-of-the-range reels, most produced to US specifications, then by all means you should link to it; if, on the other hand, you know, in your heart of hearts, that it's a poor melange of badly crafted student films, in which your acting was less than stellar, then it may be actively to your disadvantage to allow people to see it! Similarly, if you are already represented by a good agent who is solidly winning regular auditions for you with significant casting directors / have good enough personal contacts that you are meeting regularly with important industry people etc. then Claire's rule undoubtedly applies: it's vital to research the work of the people concerned as much as you can, so you can have a productive response to what they are proposing to do, when you meet. On the other hand, if you're not being allowed in to see these people in the first instance, I don't see how being signed to IMDB Pro will make any difference to that!
For these reasons, I am sure Claire is right when she says that the ability to use Pro becomes invaluable...WHEN you're in a position within your career when you can take useful advantage of it. At the end of the day, only you can judge whether it seems like it may be of real use or not at any given career point - all that can be said is that all actors face different challenges in the marketplace, and tend to respond accordingly.
