People overemphasize the benefit of on-camera external mics because mic'ing from on-camera is an inherently poor place to mic...well anything. Proper mic placement will be far more effective than using a nicer microphone. The main advantage I see to them is to narrow the pattern (e.g. if you're filming by a river but don't want the rapids to interfere with dialoge), but aside from changing the pattern and having a different EQ response, it won't have much of a positive effect. And if you're using a glidecam, it could be a complete tradeoff, at which point you have to ask yourself if its worth it.
That's not to say you should just ignore audio, since its role is incredibly important. I don't use on-camera external mics because I record sound separately and superimpose in post. Given the choice between using the built-in mic v.s. using a RODE Videomic Pro, obviously the RODE is a better option of the two; my point is simply that it isn't optimal, contrary to what many will tell you.