At the moment (meaning in the last few years!), there is no benefit of fanrank whatsoever!
There just isn't enough fan pages for anyone, so it really doesn't matter if you are a one or a ten.
Yes you should earn higher credits if you were a ten, but that depends if the person paying is willing to pay extra. These days, we are lucky if anyone pays at all!
Now you probably know, as everyone does, that a person with 2000 fans really doesn't have that many friends! He/she is gaining friends by cheating. You know it, I know it, and the advertisers know it! So if you are a 10, many people are actually less likely to want your support. They want the true account holders that have perhaps 20 - 50 REAL friends.
This forum is full of complaints that there aren't high credit fan pages for the 10's. Well, it isn't magic! Those fanpages don't just appear because people ask for them - SOMEONE has to pay for them! If no one wants to pay - you wont get them. It's that simple.
Notice that I haven't mentioned Fanslave! It isn't their fault. They also would like to earn more money. They also want more fan page likes to be purchased - BY US, THE USERS.