The marginal "help" Luke provides to his friends consists in absorbing a Washington Generals-level beatdown from Vader, thus creating a painful and humiliating diversion to distract the Sith Lord from his friends' escape. Luke showing up at Cloud City has NO negative consequences for anything the Rebellion was fighting for or anything else we can see for that matter.
And let's not forget that Luke also "helped" them pick up Lando Calrissian, the guy who eventually joined the Rebellion and BLEW UP THE SPACE STATION CARRYING THE EMPEROR. In other words, the guy who ended up winning them the war.
Great call, Yoda. With those predictive skills, you must have spent many a long, hard day at the race track.
But even this comes after Luke's training, which largely consisted of slogans like:
"Do, or do not. There is no try."
Nonsensical lines like that one make Yoda the Yogi Berra of the galaxy. He seems to have picked up an incorrect definition of the word "try" at whatever community college he attended. You cannot do without trying first. It is impossible. If he'd said "get drunk, or do not, there is no drinking" it would have made as much sense.
"WOOOOO! Pride or no pride, I will get drunk."
Or was he just telling Luke to believe in himself, to plunge ahead against all odds, regardless of appearances? After all, Yoda said the line after Luke expressed doubt in his abilities. Well, we're all for self-confidence, but Luke's whole problem was that he overestimated his own abilities, and was charging into the situation without stopping to train first. The entire rest of Luke's training revolved around breaking him of that.
Which brings us right back to the trip to Cloud City that Yoda tries to talk him out of. Wasn't that just Luke trying to seize the initiative and "do" instead of hiding on a planet with the swamp rats and doubting himself?