My point is that relying on this only makes you more likely to drop legit mail and poses no problem to the spammers.
I don't disagree with your points that: 1. It is a bad solution to rely soley upon. 2. If a spammer chooses to, it is fairly easy to bypass (although your example of buying a domain name poses some risk to a guy using trojaned boxes to send mail). 3. If everyone did reverse DNS checks there would still be spam.
I think of it like this....
At home I block ads using the hosts file. Have been doing it since forever. I use the hosts file below + my own additions): http://www.mvps.org/winhelp2002/hosts.txt
If ad banner farms would not push obvious names, and instead use extensions of the legitimate sites ( www.realfoosite.com/sleazeballad ), my way of blocking them fails. On top of that, I also accidentally block some legitimate sites once in awhile.
All that being said, I don't get pop-ups, pop-unders, or ads, and this easy-to-defeat method of blocking ads works very well, so long as I am willing to accept that I am occasionally blocking some things I wish I hadn't. ;-)