Is honesty a bad thing? Plato says it is

It depends on who you steal from. If you are stealing from the capitalist class then fire away.

Was Plato a communist or a libertarian?

Communist.

Here's an idea: don't abide by what the ancient philosophers said. Their theories are outdated and impractical, anyway.

True. And to be honest I don't think Socrates succeeds in refuting it. I mean I take his point that to live completely according to whim without any regard to others would probably be mentally damaging (even if you could get away with it) but there might be certain instances where it is beneficial to be unjust with minimal effect on your mental health. Plato being an idealist though thought the greatest good was to conceive of and partake of the ideal, therefore any imperfection in the four virtues of the ideal person was to be avoided. If you reject this idealist view, his arguments are weakened.

I take a more Humean stance on this in that I think morality relies more on passion than reason. We condemn Glaucon's position not because it is irrational (as Socrates argues) but because it is distasteful. And that I think is fine. Of course since passion is subjective, others may see no problem with Glaucon's stance, and with those people all you can really do is agree to disagree, on an intellectual level at least.

If you are only thinking about something, what has that got to with honesty regarding your own subjective opinion? However, if your best friend asked you directly if you think they are ugly, then how would you be serving yourself by lying to them if you do feel they are ugly? If they don't like the response about what you really feel, well why did they ask?  Or you could be more roundabout and tactful in answering with something like, well, you are not the most attractive looking person around but I've seen worse. Or, what I think of your looks is irrelevant regarding my friendship with you. If the friendship was solid and genuine and bound in trust, they would best appreciate and honor your honesty and virtue if asked. 

At any rate, being honest with a subjective opinion about something, may operate on a different level when you steal something that is not really yours for the taking. All that is telling us, is that we feel entitled to take something that wasn't given to us, or were meant to have if it wasn't exchanged in monetary barter or other form of exchange. That is just the way the world operates.  If you feel worthy of whatever it is you want, yet don't or can't have, stealing is ultimately not going to give us the satisfaction and enjoyment that it would be if otherwise acquired by legitimate means. There is always going to be a consequence for this, even if it is just conscience stricken. That is how the universe operates. Act authentically and the rest will follow.

What? Neither of ideologies existed back then.

He was in many respects very much an elitist in the worst sense of the word, who looked down on those who didn't have any blue blood (another simplification). And he was staunchly against anyone who stood in the way against... Not science, that didn't exist back then either, but philosophy as he meant it (a way of trying to understand and describe the world). You can call it proto-science if you'd like.