Well, I'd love to do that on about a dozen sites, where even when I put very focused keywords in ad text or content keywords, I can manage to get my ads to show up on these sites. So I'd like to do the advertise on this site option.
But...when you're limited to 25 campaigns, and you're already using them to target for other things like regions, how do you get around this?
The "Advertise on this site" link is just a channel for creating a site targetted campaign; but once you've created a site targetted campaign you can either;
a) Create mutliple Ad Groups within a single campaign, each targetting a single site, or
b) You can target multiple sites within a single Ad Group - it's just a matter of selecting the URLs akin to selecting keywords in a normal campaign...
In other words; you just need to create 1 site targetted campaign and you can then target as many sites as you like - all with different ads if you create different Ad Groups.