External Bottom Brackets

We don't and I'd rather just change the whole bb. Wasteful maybe but they ain't expensive and it's usually a 5 minute job if that. I imagine pressing bearings in and out would be a bit more labour intensive and surely the cups will have a limited lifespan if your removing/replacing the bearings which are an interference fit?

This is pretty much my view too. Once the bearings have died, chances are there is going to be some distortion to the cup, or the bearing may well have seized and slipped around in the cup, making it oversize, so pressing fresh bearings in is just a waste of time /effort.

But, some of the MTB guys swear by this method, they just pick up a shimano XT BB, and a dozen bearings, then just keep on top of them.

Remember ISIS and how often those used to go ? I've got a raceface signiture Ti ISIS BB from about 1999/2000, which cost a small fortune (£180? back then!) but its still going strong, has an obscene milage on it and has been on a SS MTB/commuter for last 7years now.
Its a 4x bearing job, and when/if anything goes wrong with it, you send it back to RF with receipt and they give you a new one/ press fresh bearings in/ whip the tech who made it.

Pretty much you get what you pay for!