I've also done this quite a bit, and have looked into the benefits of getting MNPS approval on CJ class aircraft. Depending on the state of registration it will cost you time and money. For ferry flying, the time is the problem - it can take several weeks to get the approval, and no client wants to wait. If it is a UK aircraft the money is a factor too. You'll save a bit of fuel if you can get higher, as eckhard says, but it probably isn't worth it, even if you do it a couple of times a year.