Not everything allowed is ethical, but in this case I think you are well within the bounds of ethics. I have downloaded many trials and used apps with no intention of buying, and it turns out there are a very few I HAVE purchased. Even in cases where I was certain I wouldn't buy, I'll bet the company would love to take the chance on me anyway, on the off chance I was that one exeption they could convince, or that at the least I might tell others about a good experience I had.

Anyway, the previous point about the pricing is spot on. They weren't pricing it for your needs, so it doesn't hurt anyone for you to work within their trial and dump it. They have bigger fish to fry and they can only benefit from your usage (word of mouth praise, etc); there is no downside to them if you arent in their target market.