Thinking that someone knowing your username will compromise your security is just foolish as you cannot do anything with just that UNLESS there's either a problem with the system in question (in which case you're fucked anyway)
Again, this is the problem with security nowadays: people like you that think they know security, when in fact, they don't. What can I do with your username? I can bruteforce the password. As for "there may be a problem with the security"... Really? Do you really think most sites today are secure? Just this year a stupidly easy exploit was discovered on steam which allowed you to log into any account so long as you knew the username (source). So yea, that's what you can do with a username. And who knows how long it's been there. If you think this is just a freaky occurrence, you're very mistaken. Remember Heartbleed? There are many zero-day exploits about loads of things that the public doesn't know about.
This mentality that security systems are perfect in this day and age is really fucking insane. There could be a security flaw in literally anything you use, so to say things like "it doesnt' matter if you know my username, because the system doesn't allow you to do anything with it without my password!" is utter ignorance. Honestly I feel like you must be living under a rock to say such a thing. Do you just play games all day and not read any news?
if my house is titanium (or whatever metal that you can't easily cut through) plated with 2 top of the line biometric and whatnot locks, as far as my security goes - does it really matter if you know where my house is? Unless you have a copy of my key OR know something about my locking system that lets you bypass it, you can't really do shit.
It doesn't matter how much security you have in your house—at some point, you're going to leave it, and then I'll kill you right on your doorstep.
The very definition of hashes is that they're unreversible. Brute forcing IS NOT reversing, and it is very, very time consuming it specially if you have a long password (and not based on common words).
Pedantic much? And no, hashes like MD5 can be cracked in minutes with rainbow tables.