It's not "shooting yourself in the foot" to work on a startup while working a dayjob, it's called "funding your startup with your dayjob salary". And if you don't have a pile of capital just lying around waiting to be spent, or aren't (interested in|willing to|able to) raise outside capital, it's essentially your only choice.
And why exactly should I care about "stringing along my present employer"? I show up, do the work I'm asked to do, and they pay me some amount in return. What I do on my time is my business.. If my performance drops, they are welcome to fire me. If anything, this should be seen as a call for employers to recognize their entrepreneurial minded employees and find a way to leverage their talent to do something entrepreneurial within that firm. Better yet, invest in the employees startup and tell them to get out and work on it fulltime. Now that's a win-win for everybody.
But don't think you get to call who is or isn't a "real entrepreneur" based on who has or hasn't quit their dayjob. If anything, the folks doing it "the hard way" deserve more respect and recognition than the people running around acting like raising a round was an end unto itself, and who seem to think that the point of a startup is to raise money (as opposed to making money).