I think it was a poor decision that will cause us some problems in the longer term (e.g. the 397s could be the 180s of the 2030s - a small fleet that are too short for long distance high speed services but the 125mph specification means that they aren't suitable for cascading to slower services where their length would otherwise be more appropriate).
I take the point about ordering lots of different types of trains meaning they might get them sooner (in theory), but remember that TPE didn't need to order new trains for a specific date - they aren't running a legacy fleet like Northern (who needed the 195s in time to replace the Pacers/153s before the accessibility requirements kicked in). So it didn't matter so much to TPE that they had new trains so urgently - additional capacity is clearly needed (I have friends who commute from Sheffield to Manchester and colleagues who commute from Manchester/ Stockport to Sheffield, so I regularly hear the stories about three coach 185s...) but TPE weren't in anything like as desperate a position as Northern were - if it took until 2020 for the fleet to be delivered then they'd just carry on with modern accessible 185/350s on services in the meantime.
I appreciate that the WCML services are operationally separate, so there's less of an issue with a separate fleet, but I'd rather we had one big fleet of long distance high speed trains (capable of bi-mode operation) - but First have a habit of ordering awkward little fleets (175s, 180s etc).