Below is an excerpt from an edition of MarketPosition Newsletter which gives a good explanation on why we put limits on bandwidth with option to buy more (if needed, which is very rare). We keep the peak CPU load for each server below 10% on all machines. This means if a server exceeds 10% of it's CPU resources or uses too much bandwidth, we will move clients accordingly over to another server so clients on that server don't suffer from lower performance.

Excerpt from MarketPosition (tm) Monthly Newsletter

"Be cautious of [web hosting] services that put no limits on the bandwidth per month. These kinds of services can get burned by high traffic sites overloading their servers at no additional charge. If they aren't charging for the extra bandwidth, chances are they can't afford to upgrade their equipment as activity rises. So, your site may suffer from lower performance since it shares space with these other sites. I've even encountered a couple of services that advertise unlimited bandwidth, but then when you exceed it, they tell you it's not actually "unlimited." One guy had the nerve to tell me that "yes, it's unlimited, up to 4 GB per month." Huh? Keep in mind the bandwidth issue is a moot point for most users since they do not generally exceed the 1 to 3 GB monthly limit that most hosts allow by default. You generally have to have tens of thousands of visitors per month and/or host large files for download, to exceed 3 gigabytes per month." Brent Winters, President of FirstPlace Software