As a two year veteran of the App Store I can't begin to explain how much the review system is abused.
In another thread someone did some research and correctly pointed out that I accused a competitor app of engaging in payola (i.e, paying or handing out free copies in exchange for 5 star reviews.) Maybe this was cheap of me do so, but it was also blatantly clear that my competitor was making a last-ditch effort on the part of this developer to recover from 600 1 star reviews out of 800 total reviews.
You see, generally the number of written reviews will be in proportion to the number of anonymous reviews. When this is out of whack, it's clear something is amiss. Too many anonymous five star reviews and it's payola going on. Too many one star reviews and more than likely there's something the software does that really, really annoys people.
And here's where that "trialware" in some cases would really make sense. Within this niche category of apps there are presently four different developers offering versions.
This is a hard category of software to write (it's not just another to-do list app!) And so I'll fairly say that there are only two serious competitors in this niche. The other two are fine examples of how to do a half-a$$ job. And that's just it. More than likely people will try out one of the two apps that just don't have it figured out completely and give up on the others in the store.
Trialware in many respects would eliminate this problem. One could quickly wade through the cruft and find the good apps within their niche. Payola to pad rankings would quickly become a pointless endevour. Sure trial periods would be abused, but this is the tradeoff to improve the App Store review process.