Answer: you'll lose traffic. Pretty much everyone outside this industry assumes all urls are .com, thus if you have site.pro, ppl will more than likely try to access your website with sitepro.com or they'll forget the .pro & type in site.com. This is another reason owning a .com is an important thing.
Well then if I'm not rich, I guess I just have to pick my poison. I figure if I can't get the definitive generic 1-word .COM (and frankly, who can?) then my choice is limited to some forgettable 2 word or 'brandable .COM, versus a
strong memorable keyword paired with the 'forgettable' .PRO TLD. So, then, which kind of 'forgettability' is better? The .PRO TLD extension itself, or the less-than-ideal .COM domain name such as DoggityS*****ts.COM, or IdahoAluminumCarburetors.COM? Of *course* if I can get just Carburetors.com I get that and happy day. Max memorability, max traffic. How much traffic does a good sought after service lose by going with a strong keyword .PRO vs a .COM?
There are two different things you indicate - one is that people will cling to some idea that .COM is the only domain extension worth clicking, and the other that other TLDs are not memorable.
But from experience, I know that if a website provides a good service, you quickly remember the name of it. And .PRO is not *that* hard to remember. Like "imgshack.us" , after using it a few times, I'll never forget that URL. And I just don't see why a consumer who see's a .PRO high in the SERP with a good tagline and excerpt (the content summary the search engines show), wouldn't click on a .PRO.
,PRO sounds good, and the first time a customer clicks on a .US or a .PRO or a .BIZ and finds a good service there the mystique of .COM goes down a notch. Now they'll think, hey, what am I missing by not checking out these other sites?