At the time my concerns were:
1. There's no way to limit the size of the directory the FTP dumps to on the server. If the server runs out of disk space, Bad Things(tm) happen. And we usually only figure it out after the fact.
2. It doesn't scale well to lots and lots of users.
3. It probably was against the terms and conditions of our server.
Now, I was not and am not a webdev, so these things might not be true! Especially at the time I didn't understand Linux very well at all, which made it frustrating to do anything on the server, and when Eric was around we were using a hosting service (GoDaddy). Now that we have our own server linuxbox, we can pretty much do whatever we want. And I've certainly grown as a programmer over the last few years and since I started working at Google I've had to become more adept at linux stuff.
So, thinking about it now, we might be able to limit the size of the FTP folder using this method
. I'll try to dick around with it at some point in the next month maybe.
On the scaling issue, there might be some cleverness the clients can do to make it less of an issue. And certainly it's not something we have to worry about if the number of simulations is counted in the dozens. I think the FTP protocol itself is certainly capable of scaling up enough for our needs.
I think we can tie in the FTP accounts with forum accounts, so we can be at least be sure if something gets abused who to blame. That I'm less sure of, as it would require some magic to connect the forum database with the FTP logins, which I think might be tied to system users.
If you want to start playing around with setting up a FTP based IM again, I'll see if I can get things kosher for it on the server.