Its nice to get a lot of traffic to your site, and its even cooler to get it from a popular source like twitter, facebook, stumbleupon, digg, etc because it means people are paying attention to what you’re doing. They might not like it, but they’re talking about it.
The problem with a traffic spike like that though, is that unless your server has lots of resources in reserve, the site can slow to a crawl or even crash the web server.
If you have more than one site and they’re all on the same server, now all your other stuff just went down too.
Sure, you could get a bigger, badder, faster server but that might not be cost effective unless you’re sustaining a higher level of traffic.
Rather than consolidating, it makes sense to spread the risk over multiple servers. At least that’s what I’m thinking right now, cause one of my servers is getting hammered.