A significant fraction of my income comes from Google Adsense, so this behavior would benefit me. However, I'm far more interested in the long-term health of the online advertising ecosystem, and I don't want it distorted by "click-tipping" at the advertiser's expense.
When I check my stats, I want to be confident that I've earned my money by delivering value to my advertisers -- not because a handful of well-meaning visitors decided to arbitrarily divert a few pennies from the advertiser's bank account to my own.
On the other hand, if an ad on my site is genuinely interesting to you, go ahead. After all, that's the whole point.
If I enjoy the site, actually take time to look at the ads for something I'd want to learn about more. Then I'll click an ad. Sure I could just go clicking randomly, but that would be charging the advertiser for no value. At least this way, it's a win-win.
EDIT: also if people do this alot and there is a strong morality issue or legal issue it may mean that there's a need for this type of service (tipjoy ver. 2 perhaps)