I too had this idea, but I didn't do it because it seems to violate adsense TOS- you can't create a page simply for the purpose of showing ads.
Therefore, as I read it, you're not allowed to create your own parked pages.
(let's not have a discussion about whether this rule is fair. For whatever reason, Google thinks it is a bad idea to let people do "roll-your-own" parking).
If you put up original content, then you're not parking, but maybe Google would see this instead as "search spam".
This idea is very appealing to me, but I haven't done it to date because I don't want to be breaking any rules.
Arguably, roll-your-own parking with original content (ie "minisites") could be seen as creating low-quality content with the sole intention of placing ads.
The big plus of this is that once you've got a minisite, you're well on the road to creating a proper site, and also the domain will probably increase in value.
It seems reasonable I guess, as long as the content is good quality.