Yoda and Obi-Wan are extremely willing to leave Han and Leia to die in the hands of the Empire because they want Luke to stay and train to stop Vader. Though the biggest reason Luke has a leg up on Vader, that Luke is in any position to threaten Vader is because of his association with Leia in the Rebel Alliance. I don't think Luke would've been able to pose much of a threat to Vader if he were a lower-ranking Rebel.
The way I see it, the Emperor implanted an Asskicking Equals Authority order so the institutions by themselves are no longer an inmediate threat with the Senate extinguised and the civil war ravaging and so on, only force sensitive players are able to change anything in the You Kill It, You Bought It sith enforced policy. The movies suggest the Empire is a Decapitated Army without Palpatine, the Rebel Alliance and its assets are of secondary importance there, only capable of tactical victories.
As Cracked.com would point out, Luke leaving with R2 to try to save his friends actually helps in the long run. If R2 wasn't there to fix the deactivated hyperdrive aboard the Millenium Falcon, Leia, Lando, Chewie and Threepio would have been captured and killed. In particular, Leia would've been tortured about the location of the Rebels once again before being killed, which would lead to the Rebellion's defeat. So if he didn't leave now, help them he wouldn't, and all they've fought and suffered for would have definitely been destroyed.
Jedi are not omniscient and have had a lifetime being taught that dying in the name of duty is not the worst thing that can happen. I'm guessing this leaves them numb to the fact that other people don't think that way (they typically are the ones on the front line, so it's not like they expect anything they don't do themselves).
At the Battle of Hoth, the soldiers who were in the trenches didn't do a damn thing. The whole battle was carried by the snowspeeders. For instance, after Wedge and Jansen took down an AT-AT, there was one soldier who saw it and said "Come on!", and started running towards it only for it to be destroyed by another snowspeeder a few seconds later. Even taking into account that this was all a distraction, if you took them out altogether, you wouldn't notice.
Given the harsh conditions on Hoth, the Rebels were probably planning on an Infantry assault, likely augmented by TIE fighters. The Rebel speeders had to be modified pretty drastically in order to work on Hoth, so it was likely assumed that the walkers wouldn't be of any use. The shield would prohibit Imperial bombardment, and Rouge Squadron was on hand in case any fighters/bombers were sent in. The problem (as we see) is that the walkers obviously did work, and thus outclassed 90% of what had been prepared for the battle.
The whole point of the Rebel defense on Hoth was to buy time for the Rebel members to escape. While largely unseen, I always assumed that there were some Imperial ground troops as well that were prevented from storming the trenches.
As Luke is leaving to go rescue Han and Leia, Yoda comments on how "now matters are worse." Obi-Wan replies that "That boy was our only hope." to which Yoda says, "No, there is another." Obi-Wan's been criticized for forgetting Leia, but isn't he right and Yoda wrong? The two are lamenting that they could lose Luke because he is going to face Vader. And yet, Vader already has Leia, so how is she another hope?
Partly, the line was written when Luke's sister wasn't Leia. Also, though Vader had Leia, nobody was aware Leia was Force sensitive at all.
He still had her, and wasn't going to release her, so she wasn't much of a hope, even with her Force sensitivity unknown.
During the Battle of Hoth, was there any impediment to the snowspeeders flanking the AT-ATs or coming up behind them? Every approach in the movie comes from the front, where they can be easily shot down by those massive forward batteries on the thing's head. Since AT-ATs don't seem to have rear-facing artillery, the speeders could have just flown past them from out of range, turned around, then attacked (or fired their tow cables) from behind.
Could be that the walkers came in waves, and with each successive wave the speeders had to approach head-on due to time constraints. 4 walkers don't carry enough troops to attack an entrenched enemy base, especially with walkers being destroyed on the way in.
How long does it take Yoda to train Luke to be a (admittedly partly trained) Jedi? You'd think a few months at least, the length of a Biggest Loser season say, but Luke is trained in the time it takes Han to fly through an asteroid field.
Actually, Yoda trained him for longer. The trip to Bespin, according to the EU took quite a while. The main hyperdrive was toast and the backup one was very slow.
It's also implied that they stayed on Bespin for awhile (a few days at least) before the Imperials were able to fully spring their trap.