1. slipper clutch - T120 does not have one. ......... It's a manly feature - sounds like a comment from a non rider
First, do a little basic research. A T120 does indeed have a slipper clutch. Second, a non rider!!! I probably have 200,000 miles on road bikes and have far more hours on dirt bikes than road bikes. I was a fairly proficient off-road racer and have more race trophies that walls in my shop to place them.
I have ridden most every type of bike. The bikes I had the most racing success on were all 2 strokes that have very little engine braking. I have also owned and raced several 4 stroke singles that had appreciable engine braking.
Is engine braking a bad thing? No, not unless it causes you to stall your engine, then yes. Is engine braking a huge benefit? Not particularly. It is sort of like a lazy-man's rear brake. You just back off the throttle and you slow down. Is it as effective as a rear brake? No, not even close. (unless you are a novice rider who is too lazy or doesn't know how to properly use a rear brake.)
Granted, in road riding, some engine braking can be deemed beneficial when going though a series of S turns. But it isn't that big of deal not to have it.
When racing, if you are speeding up or slowing down, the goal is usually to do it as quick as you can. Engine braking will seldom, if ever, slow you down as quick as you could by using your rear brake.
Not that this pertains to street riding, but the most effective way of using the rear brake to ride fast on a dirt bike is to give the bike a little gas while modulating the rear brake. It prevents you from locking up the rear end and gives you instant acceleration when you let off the brake. Mike Lafferty, multi-time AMA enduro champion, used to change rear brake pads at checkpoints during enduros, because he gave the bike more than just a little gas when braking.
One of the most notable features of engine braking is the pronounced sound it causes the engine to make. The same crown that likes loud pipes think that is really cool. That's fine. My take on that is "whatever".