i did the splitter off the tractor many years ago and i'm still a believer because of the reasons that many have already listed:
one less engine to maintain - it seems that I always have a carb that needs cleaning.
how many hours will you really add over the life of the tractor?
convenience - set the height so you aren't hunched over, drop to the ground to roll large rounds on.
sure, the downside is speed, but...
I built my splitter with an 18" stroke cylinder because my stoves are meant to burn 16" logs. I don't waste any time cycling up to the log before it starts the split and it doesn't have to retract as far.
a stand alone with a two speed pump is faster with no load, but i believe mine is faster when it is in the wood.
I don't have any help most of the time, so by the time i finish the split, kick the handle into retract, put the split piece where it needs to go, and grab the next piece to split, it's already waiting for me.
I had a power beyond set up on my tractor already, so it was simple to do, as opposed to a bungee on a valve, but whatever works.
I found that on my old tractor (JD770) i had to run around 2000 rpm to get a reasonable flow out of the 5.6gpm implement pump. you may need to run faster to get the power & cycle time you need if you are on a 4.6gpm.