My reading of history doesn't show the US as being the bad guy in this particular incident. Regardless, it's been, what, over 60 years? How long do we have to keep this up?
Aside from this sort of implied guilt rationale, I haven't seen anyone make any real argument for what we are getting out of this so-called "alliance". If we're going to pimp ourselves out to the world as mercenaries, we should at least be getting paid, and I don't think we are.
chucky2 also brings up a good point.
Finally, I have to wonder how many of the conservatives here who think this was a good idea would have been calling for Obama's head if anything had gone wrong (whether Obama was personally involved in the decision or not).
Speaking just for myself, I'm not so much saying this is a good idea as accepting that Obama, with the advice of his professional warriors and supposed experts, decided that this was the best thing to do, and I see no evidence to assume he is wrong.
We gain a great deal from these alliances as they give us the ability to effectively project power from well established locations thousands of miles away from the US. Our bases in Japan, Bahrain, etc have proved invaluable as staging areas for military action. You didn't really think we were 'protecting' these countries out of the goodness of our hearts, did you? Lots of them don't even want us there.
Secondly, South Korea pays the US close to $1 billion a year for the troops stationed there. While that certainly doesn't cover all of our expenses, many of those expenses are fixed ones we would have anyway. So not only do we have an insanely valuable base, the ability to influence an economically vital area of the world and act as a stabilizer, but we do in fact get paid a not insignificant sum for being there.
Granting the force projection argument, to some extent we ARE 'protecting' these countries out of the goodness of our hearts. We saw what happened in Europe when powerful nations do not protect the powerless, and we paid a price. Poland's manpower and fighting spirit was lost to us; same with Czechoslovakia's advanced weaponry. Some experts believe that Hitler could not have so quickly defeated France without the Czech arsenals, and it's undeniable that the Czech vehicles were much more reliable than their German counterparts. In fact, Czech armored vehicles fought in German service until the surrender; even when long obsolete, the quality of their design and execution ensured that they would be recycled and used as artillery tractors or self-propelled gun platforms long after progress made them obsolete. If allowed, a relatively weak nation can pick off even weaker nations, becoming progressively stronger and taking progressively stronger neighbors. Considering that Communism had just gobbled half of Europe, I don't this was an exaggerated threat then, and given North Korea's willingness to let its own people starve to obtain nukes I don't think this is an exaggerated threat today.
Why would our several thousand troops be wiped out? Certainly you can see the value of having an already established supply, command, control, and defensive system set up before the start of hostilities instead of having to make it on the fly.
Having those troops already there would make things much, much, much better in a conflict with North Korea.
There's some value to this as well. US troops were originally left there to stiffen the South Koreans, demonstrate our support for our ally, and ensure that if the Reds invaded, they would automatically be attacking Americans and therefore the President of the moment could immediately react. I doubt the South Korean armed forces need any stiffening today, but the other reasons are still valid. If attacking South Korea means also attacking America, that has significant deterrent advantage if any sanity remains in North Korea.