There is no good reason to do so actually.
A machine that can only go to 10.7 and not directly to 10.8 is just better off remaining on 10.6.8 for performance and running older PPC based software.
In fact I suggest any Intel Mac that's older than 3 years (especially out of AppleCare) to remain on the speedy 10.6.8
Eventually hardware will die and one can buy the newest machine with the newest operating system in one package, along with all new software etc.
Apple is maintaining support for 10.6.8 because of AppStore, as it can be used as a attack vector if it's not updated I assume.