I completely agree on the questionable utility of asking Glock to make a G26 with one extra round in the magazine, but, if that's what a large customer wants and Glock is willing to produce it, I feel "no harm, no foul." My biggest issue is there are a metric ass-ton of G26 mags on the market that WON'T, from what we can tell, fit in this new G26 - which sounds like a logistical nightmare in the long run.
On the modularity piece, I see it as a plus for an agency that wants a one-gun solution - procure a number of FCUs, outfit them with FS slides and grips for patrol (grip "size" determined by shooter and FI as far as S, M, or L module); Compact slides and grips for plain clothes (same drill on specific sizes); and SC slides/grip modules for back up - all feeding off a common magazine for reloads. For individual shooters, I think the modularity (outside of picking the grip size that "fits" best) is pretty minimal, based on the cost of the conversions...