Right now, I only use Adwords for the main site and leave the others to fend for themselves in the organic SERPs, which they do very well. I have been thinking I should use Adwords on a few of them though, but there is a good chance that they could share some of the same keywords. I could do this in one account, but then have the possibility of not showing the right site to the buyer. Also more than likely, I couldn't even get them to show reasonably equally unless I added them as new ads in existing Ad Groups, but that shoots easy performance tracking in the head.
The logical way to do it would be to have seperate accounts, but then you have to worry about Google gettng mad about the overlap on a few terms and killing everything.
Ultimately, this seems to only drive up costs and benefit the google, not the consumer or the advertisers who are only using one domain.
I understand using the tactic from a business and marketing standpoint, but at its root it seems unethical.
I really don't think that there is anything special about googles serps that would imply that having more than one position is somehow unethical.