If you just set them up and then leave them they drift out of google rankings for a while, but then will come back after a year or so - and sooner with a little bit of link building.
They need to have an exact match domain and decent on page SEO to do this.
One advantage of building a quick minisite is that the content you write can give adsense plenty of hints as to which ads they serve up. This often leads to more relevant ads than parking.
With parked domains on niche subjects or geo domains, often the ads are so off target that it's hardly worth bothering.
And finally a minisite that's on topic and indexed is that much more useful to a potential buyer.
Try to this about a page and subject structure that might be valuable to an end user buyer so that they can re-use some of the goodwill and PR in your site post-acquisition.
Conclusion
If you believe in a domain's long term value then creating a minisite for it won't make you rich, but it will enhance the value of the domain if done right and it will make you a few quid along the way.
