I'm not arguing, I find this sort of stuff fascinating believe or not and it's always good to bounce ideas off of people. I know for a fact that if I hadn't had the support of my fellow class mates at uni to throw ideas around, none of us would have done as well as we did in the end
Fragmentation can also occur when the disk drive is particularly full. Your are right though, most modern OS's and file systems will sensibly organise data and will periodically attempt to de-fragment what it can. I still think there is an issue with lots of small bits of data being collected from lots of different places on the hard drive though. If you think about it, information relating to the user is probably in one db table, then there is data about the post, the thread, avatars, logos, different scripts and includes, icons and so on. Its a big toll
However, even a crappy little netbook (the tiny 10" things with no optical drive) could probably run this without much issue. The problem is when you start tacking on lots of forums and other websites and resource intensive applications. The end result is that the poor hard drive takes one hell of a beating in terms of read / write requests where as data throughput is probably 10-20% of its maximum capacity.
As I was writing this, your second post has arrived (email alerts ftw). I was originally talking about file systems as fragmentation in the database is usually not significant for a forum - your are correct in that DELETE requests are a complete minority. I would suggest probably 80%+ is SELECT, the rest being UPDATE and INSERT a majority of which is probably UPDATE.
I don't think there is much that can be done to make the site faster since Farjo doesn't control the server and its setup - they rent hosting. It's just an interesting topic to discuss in a hypothetical world