There are arguments for and against decasing and what's best for you depends on your point of view.
I pretty much always mount my PC bits on a wood board, which I then mount inside the cab.
EDIT: I thought about it and the only cab where I used a case was my first. NTTAWWT.-There's no chance of dropping a tool or bumping the mobo or any other parts of the pc with an elbow.
-If it needs servicing, disconnect and bring to the bench.
-no altering of airflow designs to crucial pc components.
-heat rises most cabs have a bottom inflow (in this case a DK cab has an intake in the bottom of back cover and the handle openings at the top act as outlets by original design) set pc case on bottom of cab and forget about fancy fans and wiring.
If you bump a board mounted inside your cab with an elbow, then you are doing it wrong.
For someone like me, who has multiple cabs and regularly has to move one to get at another, having a loose PC in a cab is far more likely to cause a problem than having a PC mounted on a board.
I build with real arcade monitors and JAMMA interfaces, and my test bench uses (surprise, surprise) a real arcade monitor and JAMMA interface, so it is way easier to just pull the slab. Also easier to diagnose control issues.
I have never had a problem with heat build-up in a cab, even though people talk about it like it happens all the time. I suspect it's like the "people die discharging monitors" discussions .
Whatever works best for your application is the right answer.