A micro-fleet for a specific route is relatively easy to manage, as only certain depots will sign both route and stock. Micro-fleets are more of a problem when used to make up the numbers as part of a larger common-use fleet, such as if Northern had a handful of 175s which were used alongside the rest of their DMUs.
A handful of 170s dedicated to HOWL which is only operated by a handful of crew depots avoids most of those micro-fleet issues. 158s can only run on the line if the suspension airbags are deflated, so aren't suitable for that route. The low linespeed isn't ideal for 170s, mind.
The plan is for the HOWL units to interwork with the Crewe - Shrewsbury locals. Put the two together and you have four depots involved.
Heart of Wales is currently drivers from Carmarthen & Shrewsbury, guards from Carmarthen, Shrewsbury and Crewe.
Crewe - Shrewsbury local involves drivers and guards from Crewe, Shrewsbury & Chester.
If you were going to use a microfleet just for those routes, it does seem like a strange one to use. As nice as the 170s will be, there are cheaper fleets available that are also more suited to spending most of their day below 60mph.
There would also be a big question mark over what units will work the other services that are earmarked for 170s, namely the West Wales branches. They need something to work them, there aren't enough new units coming in. Whatever works them is surely going to work the HOWL as well.
The only plausible scenario I could see for the 170s to leave before the end of the franchise would be if some of the 175s were kept on instead. They don't yet have a new home, the new refurbishments have been done to a very high standard and staff don't need any training on them. But surely it that was the case then all the 170s would go, you wouldn't need a microfleet. 175s can and have worked everywhere the 170s are planned to go.