Pros
I have yet to encounter a good reason.
Cons
They are saturated with tutors. I signed up with Tutor.com in December of last year. Every Friday for 3 months I signed in religiously to schedule hours. There were no hours available, ever. Occasionally, they allowed new probationary tutors to schedule hours on Wednesdays, which is the day that higher level tutors can schedule hours. Even then, there were no hours available except for one instance when there were two or so hours available at like 3:00 AM on a Sunday morning. I had school that morning, so I couldn't take those hours. In short, I was "active" with Tutor.com for three months and was not able to get a single hour scheduled. Not one. What really discouraged me, is that I wasn't even able to get hours when they allowed me to "go to the front of the pack" by giving me the scheduling privileges of a high level tutor. That showed me that I wouldn't be able to get hours even if I had been there for years. This is all very frustrating for me, because I spent a lot of time preparing for and then taking the two required subject tests (yes, you have to test into two subjects, not one like they pretend), taking the proctored session, and then logging in repeatedly to try and get hours.