The documentation is... less than optimal. The website/wiki can be confusing. Feature sets are not exactly stable (new features require code changes that sometimes break old scripts or features that existed in past versions are lost). Overall stability varies between versions.
The 3d industry was using commercial programs ever since because they existed long before blender was usable (I remember a time where it didn't have an undo function yet). When and why should they suddenly decide "hey, let's all throw our years of experience with commercial software xyz into the bins and start learning this badly documented free open source tool."?
Not very likely I'd say...
This might change over time with people getting started in the 3D industry as freelancers or small startups that start out with blender knowledge instead of commercial app knowledge. To people just wanting to learn 3D stuff for themselves Blender is very attractive since it's free and has a growing feature set.
Blender has its pitfalls imho but considering that it's free it is an awesome piece of software and I use it myself. I find it very hard to go away from it though and learn something more established. If you plan to ever do this professionally (as in job not hobby - not implying blender could not get you professional quality results) I'd go for something commercial.
Also not very widespread as far as I know is Modo, but I heard good things about it and afaik ID Software used if for Rage.
Yes I sometimes play hack and slay games with the tablet (have not yet played Diablo 3 yet and don't intend to). Mine is A4 sized but A5 would work too for me. I also tried to play Quake 3 and Call of Duty with the tablet, but for that you have to switch the pen to mouse mode and it's not really an advantage over the mouse. I'd say using the pen I'm a little bit better with the railgun in Quake 3 but a fair bit worse in everything else.
For Diablo and the like I find the pen to be a bit less strainfull on my arm.
And yes, the mouse that came with the tablet is total crap and I never used it.
Personally I'd stay away from all books that have "digital painting" or similar in the title because all that I know are crap. Putting the focus on the DIGITAL in the title imho is usually a sign of failing to make the correct destinction between painting problems and problems with the digital workflow.
Just my two cents...
p.s.: a few thousand hours of practice also will get you a long way, so... happy painting!