I've never done this but it makes sense. Actually I'd be interested on how this works in practice given I am somewhat naive in terms of steaming.
I'd always thought of steaming as a good way to keep nutrients in and perfect for when you want your vegetables to retain flavour and crunchiness by virtue of the fact I thought you didn't have to steam for long. With a potato for mashing, if boiling, you want to boil it for a long time to get it in a state of being mashable.
Does the same apply for steaming a potato to mash? i.e. do you just steam it for a lot longer than you might otherwise, or cut it up a lot smaller to quicken the process. I would have thought this was terribly inefficient (heat wastage) way of preparing a potato for mashing when the end result is going to be more about the flavour of what you put in the mash; butter, cream, seasoning etc. Assuming of course you're not going for a skins on rustic crushed potato type mash, where its more about the flavour of the potato.