Okay, back to the pre and color tags. Steve, I'd have responded to your PM, but I get the message "Sorry, but the administrator has prevented you from sending private messages." when I try. So I'll discuss it here.
Obviously, yeah, going back and fixing all of the old posts would take a heck of a lot of time and processing power and would probably bring the forum offline for goodness knows how long. That's why it'd be better to program in the missing tags than to recrunch the old data.
There were only a handful of color tags, from memory black, white, red, orange, yellow, green, blue, purple, and I think brown. Programatically, these are just shortcuts for the 'new' color= tag.
And in the same way, the pre tag could more or less be handled the same way, but as a size= tag. (With a small size, probably around 6 or 7 pt.) Though ideally the pre tag would also ensure that there is a newline before the open tag and after the close tag. And it would also always ensure the use of a fixed-pitch font. So a crappy pre tag could be programmed in quickly. A proper pre tag would take a little more work. But either is better than no pre tag at all.
Now, as for why support for these tags is so important, look at posts such as number 327 in
this thread. Compare the weird giant triple-spaced Cedrik to the one in my avatar. (Which is how he's supposed to look.) This is just one example of how posts that had the pre tag in them have become ... monsters. They're barely readable. I'm not sure why they're being triple-spaced, but it's nasty.
Granted, the pre tag is of a lot higher importance here than the old color tags. Missing color info isn't even remotely a show stopper. But anyone reading old posts is going to have a heck of a time dealing with the old pre tags, so I'd rate that one up higher in priority.