I have several shared accounts with different companies and one dedicated server for my high traffic forums seperate shared accounts to get different c class ip's so inlinks from them will have more value hope this is what you asked this question for
I use dreamhost shared hosting. They have several different servers so you can spread your site around them. They give me tons of bandwidth, too.
I have multiple dedicated servers and IPs in various Class C's. I'm sure you find people here with every imaginable setup.
There are other reasons, but I would agree to some extent--especially if you just have a handful of sites.
Just out of interest, at what point would people consider traffic to be mega in terms of bandwidth/views/etc ?
I would say that mega traffic starts where the big server and web hosting providers place there bandwidth limits. i guess over 3000gb a month or for server rentals, not sure on the shared / vps stuff.