I agree with Shane that's it's not that common to skip, but I have to say I am not sure why. With all the personal workouts these kids do now if a team really wants to see Murray they will. The argument with personal workouts was always they're scripted, they do only the throws that kid does well, not the ones he struggles with, they spend weeks drilling for it, so it's not a "fair representation." but we all also know that teams are well aware of it and will often ask for a 2nd non scripted workout, all the way back to Alex Smith that happened for example.
My big concern with combine and I say this every year is the morons who test positive. If you can't stay clean long enough to test clean at the combine when you've known for ages when it is, and when they will test you, i don't trust you.
We saw yet another example of that the other day when Randy Gregory was suspended yet again by the NFL. He was one of the guys who had positive tests in college, did horribly in the personality profiles they do, and then tested positive at the combine.
The advantage of the combine is teams can see all the prospects in one place. The disadvantage has always been that it's not an ideal setting. What if you have the flu or a bad cold, you have to decide do I work out sick, or do I pass and risk them thinking I'm ducking something. You have guys often standing around waiting their turn, I remember Deion on tv once saying he was terrified he was going to pull a hammy when he ran his famous combine 40 because you have to wait and you just keep stretching and warming up but it's such an unusual format for them to run in it scared him.