If I were going to get a real small shotgun I would want it to be something like this:
No loss of magazine capacity. One can still run 5-20 round magazines, with 8-10 rounders being the most practical in my experience.
Can fold the stock for a pretty trim package if desired. Even with the stock out it is relatively small
About 1000X more "usable" (getting on target quickly, follow up shots, accuracy, etc) with the stock deployed than a PGO shotgun.
Also since a lot of the attraction to PGO seems to be fun or cool factor it blows them away in that too.
The one drawback is it is not real cheap.
One thing I've never understood about the Serbu is why everyone is so excited about it being an AOW and requiring only a $5 tax stamp. I understand if do to local laws one could have an AOW but not an SBS but otherwise it is a lot cheaper to build a much more useful SBS including the $200 tax than to buy a Serbu and pay a $5 tax.
To answer the OP, yeah guns like that are door breachers for serious users, range toys for others, and a HD weapon for those who are either misinformed or what to handicap themselves. Their main practical use is as a door breacher, although the FBI HRT apparently has tastes closer to mine. Here is the breacher than Tromix built for them. (I believe it was not yet completed at the time these pics were taken).