If the on camera audio is made to be usable with the help of decent mic on the camera such as the Videomic Pro etc it gives you a better opportunity to capture that ambient audio. There has been many occasions that our audio tech has screwed up a part of a shot due to technical difficulties and that clean on camera audio has saved our asses.
Lets be frank, the whole video thing is designed to be an add-on to a stills camera. It wasn't designed for b-roll or anything else. Just basic audio.
Now the video has been adapted from its early incarnations in the 5D2 (initially auto exposure NTSC only) and 500D (not full frame rate at 1080, auto exposure, no mic input) and only really progressed through firmware updates and the likes of Magic Lantern pushing the boundaries.
Now it's quite good.
On the subject of magic lantern, apparantly you can set it up so the L channel is what comes through the mic input and the r channel is what is coming through the front mic on camera. If this fix works on the 60D then that well may be the most cost effective way forward.
I have a sennheiser MKE300E (hangover from my vx1000 days) for the occassions I just need a little better than front mic quality audio, and I also have a rycote microwindjammer on the cameras built in mic, to make it that little bit more usable should push come to almighty shove and I've only the camera on me.
The point is that it takes more than a decent mic to get decent sound, and in actual fact, on top of the camera, next to your breathing, handling and motor noises is just about the worst place you could put it.
I already had a decent camcorder mic sitting on the shelf, for the little difference in cost between the rode video mic and the rode NTG-2 the OP was sold the right thing. The practicality of the video mic is very quickly exposed in use.