I see two explanations that would make sense:
Yoda was too weak, physically, to be of any use. He was older, and living on Dagobah had surely taken its toll on his physical strength. His connection with the Force might have been stronger than it was during the Clone Wars, but his body certainly could have only gotten weaker. There is no guarantee he would have been of any use in a fight with blasters and light sabers, and possibly could have even been a hindrance.
Yoda would have made the probability of their escape even less likely. Yoda was in hiding on Dagobah, masked by the planet's strong Dark Side aura. We can assume not even the Emperor was aware of Luke's training with him. Vader and Palpatine assumed Luke had been training on his own after Obi-Wan's death, and likely thought that without a teacher, he could easily be turned to the Dark Side. Remember: Vader LET Luke and friends escape Bespin. Had he wanted them dead from the beginning, they would be dead. Had Yoda confronted Vader with Luke, Vader's plan probably would have quickly shifted from trying to turn Luke to wanting to kill them both as fast as possible.
It's also not impossible that a part of Yoda wanted Leia to be rescued, should Luke eventually fall to the Dark Side.
Keeping that in mind, let's look at the possible outcomes:
Yoda and Luke stay: Luke is the last hope, will be more easily turned because of grief and anger that his friends were killed/captured.
Yoda and Luke go: Both are probably killed by Vader. Leia captured. Hope is lost.
Yoda stays, Luke goes: Luke confronts Vader. Leia given a chance to escape. Vader might not kill his son. Yoda chills in swamp hut with his new flashlight toy.
Yoda is gambling on what from his perspective seems to be the only way to both keep Luke far away from the Dark Side and keep his own presence hidden. He surely remembers what happened the last time a Skywalker lost somebody they thought they could save, and as macabre as it sounds, he probably thinks Luke is better off dead than giving the Emperor Darth Vader 2.0.