Having a train that can haul cookies is always going to be a positive thing. The Superliners IIRC are good up to 100 MPH, but you can't find anyplace where they run on track that's good for 100 MPH, heck I don't think for any route they run on primarily you can find a place where they run at 90. So even though they are rarely, if ever stretched to their limit, having the capacity there isn't a bad thing. Having the ability to do something is not a bad thing, it allows you flexibility for the future.
As far as not buying the motors to match the set, it's pretty clear why they didn't do that. Under current conditions it's not necessary. Since they're shooting for 110 MPH operation, and that is within the capabilities of a P-42, it doesn't make too much sense to go out and buy these. You would have to buy at least three motors (two for the road, and one for maintenance rotation) plus cab cars to be independent from Amtrak. But with the many moving parts, maintenance, etc. that goes into an engine vs. cars it makes sense to keep using Amtrak's motor power.