Since I think Hyatt is the only other program with LT status, is there any reason why you would switch to SPG, IHG or Hilton? The get LT Plat, you must have found that Marriott had good hotels in the places you like to stay at prices you felt were worth paying. I'm working towards LT Plat myself and when I achieve it, I plan to use it!
LT Plat means that I won't have to make mattress runs to fill out my 75 night requirement. I'll also make more stays on points since I won't need to accumulate more EQs. I think the best thing about LT plat will come when I'm no longer travelling for business and don't have enough nights to obtain status with anyone.
Aside from that, Marriott won my loyalty with it's performance -- LT plat would be worthless if I didn't like to stay with them.