Hello fitzz,
Well here is what I have seen. Of course the advantage is you can change reel position at any time for any reason. The disadvantage is if using it to often corks start to wear causing the seat to become loose and the next thing you know your stuffing business cards under the seat to get it firm again. The other thing is they tend to be a bit bulky in the hand. And if left in one position then tend to get stuck in one position anyway. When it comes to doing cork rebuilds on rods swapping a sliding seat for a fixed seat is more often than not the reason it is being done. Sorry to be a dark rain cloud but as you can see I am not a big fan. If time is spent putting a fixed seat in a position that suits your style the best there really is no reason to move it. Some might say well I can use my levelwind and pin reel on the same rod but really that's not right ether and here is why. The guide spacing on a pin rod and casting rod are different. Even more important is the spine on the rod is not the same on the two different rods. Can it be done of course it can, is it right, the answer is no. I hope this helps and is only my opinion and others might disagree and at the end of the day it is your call to make. If I can help further I am pleased to do so. Good fishing.
Regards,
Mark