According to E.G., downshifting w/o the clutch is generally ok as the forces on the trans are not great enough to do much damage, the rear tire should soak up most of the shock w/a slight skid. Upshifting w/o the clutch produces more shock to the trans and is more detrimental.
As far as a faster takeoff it's just basic physics. If you are spinning you are wasting time. Using the clutch well can keep you just on the brink of spinning which is what u want,
If I'm on a hill and need to nail a lower gear to pull it, I'm not worrying about the clutch, I'm gonna make the hill, then rebuild it when I have to.
Generally speaking, I think using the clutch effectively should always give me an advantage, more control. I have a 250 and slip it all the time in tight stuff. Also keeps the revs where I want them. It's fast enough so I can clutch it on the upshifts also. I tend not to use it on downshifts just because I believe it causes minimal shock to the trans.
I'm just a trail/woods rider so take it for what it's worth. I know when I owned a 125 at 16 yrs old, upshifting by backing off the throttle (no clutch) made all the difference in the world, the 250 isn't much of an issue, not to mention I'm 20 years older and slower now.
Joe Chief
Having a short clutch lever helps me to use it more. I broke 1/2 of mine off in a spill before I found an aftermarket, sanded it smooth, and it works like a dream.