If you look at interviews with pros, a lot of them tend to use Av mode for most situations. When they say to only use manual modes, they usually refer to using Av/Tv/M, and staying away from the Auto mode. Using Av mode offers more flexibility, especially when the light changes rapidly. With M mode, if a cloud passes overhead while you're setting up your shot, you're screwed. With Av mode, the camera will compensate. Then use the Ev +/- feature to compensate for camera error, especially when there's back lighting, or if the sun is playing funny tricks that the camera can't understand.
When I started off with the T2i a few years ago, I also tried to stick in M mode, and I wasn't ready for it. I then switched to using mostly the Av mode, except in cases where it didn't work. Sometimes I need a specific shutter speed / aperture / ISO that the camera won't let me choose. For example, I know I can hand-hold my Tamron SP 70-300 VC USD at 1/60 shutter speed with 99% success rate at 300mm thanks to the USD. But if I use Tv mode, it might give me too much or too little aperture, and the depth of field isn't what I'm looking at for the shot. Then I switch to M mode, and usually let the auto-ISO on.
In some cases, such as astral photography, light painting, fireworks, etc, you're not exposing for what the camera can see, so I definitely go in full M mode for those, including manual ISO.
After 2 years and roughly 8000 photos of sticking mostly with Av mode and examining what worked and didn't work, I'm now starting to have a general idea of what M settings will work, and I'm starting to use it more often.
Any true professional will tell you that the important is to get the shot. If Av permits you to take shots that you wouldn't be able to pull off in M mode, go with it. Likewise, if there's shots you can only achieve with M mode, go for it.
Cameras: Canon EOS 6D, Canon EOS Rebel T3i, Canon EOS Rebel T2i, Canon S90
Lenses: Tamron: SP 24-70mm F/2.8 Di VC USD, SP 70-300mm F/4-5.6 Di VC USD, Rokinon: 8mm Fisheye cine, Canon: EF-S 18-55mm f/3.5-5.6 IS Lens, 75-300mm f/4.0-5.6 III, and EF 50mm f/1.8 II
Retired camera: Fujifilm Finepix s700