Fair point on maintenance and knowledge.
I was looking at it purely from the perspective of a service matched to passenger demand, and that could easily be delivered now without the use of HSTs.
Let's see what they do, but I'm sure we'd agree that HSTs zooming around all over the place while 170s are sat in depots wouldn't be the best move.
If it's purely on a cost basis, I'm not sure that the HSTs are a huge difference to 170s in fuel consumption. They coast a lot more because they get up to speed much quicker (about 2 mins less to reach 100mph), and on gradients they can be eased back to hold speed while 170s often won't even make linespeed in full power.
If ScotRail do indeed reduce HST diagrams, then at least they should rotate power cars rather than stick to a select few.