I've posted a huge rant about this before ... but I'd like to remind everyone again that M brings you absolutely nothing over Av or Tv. In non-flash situations M brings you nothing but slower shooting. M is there for situations where the internal light meter cannot be trusted.
Putting your camera in M, selecting an aperture of f/4 and rotating the other wheel until the exposure indicator is dead in the middle of the graph is the exact same thing as putting your camera in Av and half-depressing the shutter. The difference of course, is that it takes your camera a fraction of a second to do it while it takes you a lot longer than that.
Putting your camera in M, selecting a shutter speed of 1/125 and rotating the other wheel until the exposure indicator is dead in the middle of the graph is the exact same thing as putting your camera in Tv and half-depressing the shutter. The difference of course, is that it takes your camera a fraction of a second to do it while it takes you a lot longer than that.
For those situations where you need to under or overexpose (as compared to what the internal light-meter thinks is correct) ... in Av or Tv you would simply rotate the exposure compensation dial (which would be exactly what you'd have to do in M mode as well, only again, this would take less time).
As I mentioned in my rant, I understand that some people will have some situations that would necessitate M ... I was using a lot of flash indoors this weekend and set it on M for a few hours for instance ... but for 98% of the photos I see posted on POTN ... Av or Tv would be all that's necessary.
And for the record, being in M mode doesn't "teach you about exposure" ... it does little more than teach you an extremely inefficient method of shooting.
Bill