Well, if it took longer to transform while turbo it would stick to the show...
I mean, they sometimes had to struggle to push themselves to go SSJ, like Gohan in ROSAT, but it takes a good bit longer.
That way, people would desire much more to go that little bit further and transform quicker. But in a pinch it could be very handy if your PL is smaller than the other guys, and you need to get more power.
Like if he's kicking your ass, and you know you could do it with a bit more power, but can't transform, you find a place to hide turbo, and go SSJ/whatever. And maybe instead of taking longer it could just absorb Ki?