Again, they can configure it so that the already registered users get a higher amount of blocks. Also, "claim blocks" are not individual blocks. It's a radius effect, let's say you mark an area 100 x 100, you can build as high as you want, or as deep as you want. Anything in said radius will be protected. As far as the towny issue goes, I'm not too familiar with the plugin itself. But if I remember correctly you can add "no claim" zones  to the plugin I suggested. It also makes additional anti-grief plugins unnecessary, lowering the server load.

Not everyone wants to join a town, or make one (like myself). I suggested this plugin for me and my fellow lone wolfs. Also, it's not just so we can protect our houses. If you read the plugin information, it protects against EVERY form of griefing. No one will be able to steal your horses, or take their saddles, it prevents people from messing with your hard work. People won't be able to mess with your redstone repeaters, or anything like that matter. Most servers that I've been staff on, had this plugin. If it didn't, I would make them implement it. This plugin would stop all the griefing, as it did on those servers.  (Not that there's THAT much going on, but new players often do grief every house they come across.)