Thumbs-up are limited so that a few "cheerleaders" or a clique can't artificially inflate each others' Rep score or perceived popularity.
It's to maintain a firm connection with reality.
Thumbs down are limited too, but the % throttle is different.
The limits are calculated by %. You can't be more than X% of someone's upvotes, or more than Y% of their downvotes.
So as more people up/downvote a person, it allows his "cheerleaders" to upvote a few more times, or his mortal enemies to downvote a few more times.
If a person is truly deserving of downvotes, you likely won't have any problem giving him as many downvotes as you desire. Same for upvotes.
That's the theory behind the system, and it works. I've seen it work countless times.
P.S. It works even better the more people participate in the system. It's just like voting -- if you don't get out and vote, then other motivated people get to decide who runs the country! Or, in the case of CathInfo, they get to decide what is perceived as good and bad.
Of course, in the case of voting and America, your vote can be manipulated by electronic voting machines, etc. but on CathInfo, your vote is secure and counted.
So get out and rock the vote! :)