thing about 100mbit is you most likely need ddos protection because someone could eat 100 or more gigs bw before you even noticed. Plus the attacks are evolving where they arent picked up by anomoly detectors or they dont even care about knocking you offline instead just eat your bandwidth. Below is a wierd udp/syn flood I had got a few hours and didnt even notice it, I had softlayer techs add it to the ddos guard manually and it stopped. Had I not noticed id been screwed for the month.
I never would settle for 10 mbit pipes but looking at my bandwidth day to day doesnt look like I would even max it out and I have semi high traffic sites on both servers. I guess it depends what you host, if you hosted regular web sites, forums, and such you could do it with 10 mbit. If you have sites with files for download and its halfway popular you would saturate it.
I looked into this a lot as well choosing my last servers. I say in reality if you have a box that uses under 2000 gb a month you could handle 10mbit well, may not be 100% at all times but nothing too noticable.