If the transmission has been properly serviced and the shift characteristics are currently good, I see no reason to spend $1500+ just for the heck of it. Fix the leak and keep the cash in your pocket. You might want to have them look at the rear crankshaft seal while the trans' is out. If it shows any signs of leaking, now is the time to address it. Additional labor should be around 1.5-2.0 hours at the most. I re-built a 722.4 transmission as a favor for a friend when the car had 150,000 miles on it. It currently has 415,000+ miles on it, and other than a slight harsh engagement into DRIVE it still shifts fine. I've never replaced a tranmission because of leaks.