why do some dislike clutches?

ive been testing loads of motors recently in a ofna gtp for the purpose of oval racing. we currently run all nitro cars in the series but i have done a brushless conversion and i am now running in the series against the nitro.
i started with a 7xl then down to 8xl and now settled on the 9xl which matches the speed of the nitro,s to keep it on a level.
on all the testing its been dirrect pinion to spur with lots of wear to the pinion. i have just recently installed the elektri clutch and the results are great. wear problem gone but the best thing is i have gained a third extra run time from my cells.