My first cab was a 4-player Neo-Geo which I bought back in 2003. The only modification to the original cab (apart from shoving a MAME pc inside) was I cut out a new CP top piece in the same shape as the old, so I could put new joysticks and new button layouts to my specs.
This time round, I am going to build one from scratch. Partially for the challenge, and partially because you can't get even an old half-busted cab in Australia for anything less than $1000, seemingly. So, from a financial point, it makes sense to build your own.
More importantly, building from scratch means the cab is FULLY customised to your requirements / tastes / wants.