While I don't have a problem with the 'I want to actually try a demo that doesn't suck, so i'll DL a full version and give it a proper go' philosophy (which is what I did for World in Conflict and Hearts of Iron 2, both of which I love and now own retail versions of, and both of which had demos not representative of the full game), the issue comes from the 'well, it's a fun game, but not worth paying for so i'll keep playing but won't actually buy it' crap, which i've also done.
The original intention CAN be morally excused, even if it's a grey area (after all, the publisher DOES profit from it), the 'ooops' result can't, and since doing the former probably leads to the latter more so than not the whole exercise is easily open to critisism. More so since people will simply use it as an excuse to be a plain dirty pirate.
The real problem is that we can argue all day about this irrelivent subset of grey-area piracy, but at the same time I buy most of my games second hand. Sure i'm handing over money and the store loves me for it, but the end result to the publisher is the same, which, in theory, makes me just as bad.
This is a whole seperate argument, but one that I feel is a lot more relevent than the 'but I only pirate to try' crap that we've seen so many times before.