One, I think that everyone forgot that at the end, Shaggy had that whole part in which he realized that he didn't need the hypnosis to deal with the bad guys, two, of course he didn't haxstomp any of the bikers in that fight, in that movie, Shaggy literally didn't have any of his powers.
Two, Shaggy is capable of killing, in the Scooby Doo vs KISS movie, Shaggy and Scooby kill one space alien flying thingy that they comically turn into a roasted chicken (It's murder but hey, gag).
Three, Shaggy can read minds, as shown in the comics, is literally here, it was on his respect thread that no one tried to check, but I will tell you what happened.
"Duck!" Shaggy shouted as the bullets went through the window.
'He can say that again!' Scooby thought. (It was into the thought balloon, so yeah, Scooby didn't think aloud)
"Duck!" Shaggy shouted a second time.
Four, Shaggy when able is an asshole with his enemies, for example, in Scooby Doo get a clue, he constantly tormented two guys who invaded his house after he made them small with one of his uncle's inventions.
Five, the dude had to fight a immortal samurai of thousand years old without the whole hypnosis thing.
Yeah, Shrek randomly saying bad won't do that.
Also guys, Shaggy has the Yellow Ring of fear, which is powered by his own fear, do you really think that Shrek scaring him really will make things easier to Shrek?