The problem is that however enthusiastic/competent you are, if you're a one man band you can't be halfway through pulling a gearbox and the phone rings? And it is NOT a good thing to greet customers looking like you work in a scrapyard!
A case can be made if/when you need something sorting quickly and you're confident of your diagnosis. My mechanic is efficient and cheap 'but' if I'm waiting for something simple like pads/disc and he's called out on a breakdown it quite frustrating. Or, as recently happened, I'm booked in for a 'belt' and he's still stuck on a BMW with stripped head bolts?
Recently, in the last few years I've backed right off of DIY. It's not ability that stops me from working on cars, but my reluctance to spend any more money on specialist tools, which seem to be required for every job.