Not the first time S&W has run compact & subcompact models in the same general model line. The compact 3rd gen's were run alongside the subcompact Chiefs Special models. They only ran approx half an inch less in barrel length difference, too, and with only a round less in mag capacity compared to the slightly larger compact guns.
I see this as pretty much the same concept, and S&W obviously feels there's a market for a slightly slimmer & shorter M&P model.
If this new trigger is as good as I've been hearing (for a plastic gun), and the practical accuracy is similar to the other M&P's some of the other guys and I own and have been using, it'll be a welcome addition to the M&P concept.
It might not be as nice as the triggers in my SW99's (the TDA-type actions), and it may run half a pound heavier than the trigger of the 99 variation Walther is selling as the PPQ ... but Walther doesn't make a compact or subcompact in the PPQ line, and the Shield is smaller than the M&P compact series.

Being in the M&P model line, it ought to be a bit less complex than the PPS, too (from a an armorer's perspective).
I'll have to get one in my hands, though, to really assess what it's like. The LE distributor said they can't order the commercial release models, and they can't order the LE version until S&W has them in inventory.