SonosNet uses the same frequencies as 2.4GHz WiFi so that would not necessarily deal directly with a crowded WiFi.  There may still be performance gains from using SonosNet, of course.

There are arguments for and against allowing Sonos portable speakers to join SonosNet.  But it’s a reasonable request to make.

Not counting the home theater surrounds and subs, only 5/15 of our units are wireless. Of those 5, 3  are portable speakers. I notice they don’t don’t respond as quickly as the wireless non-portable units which use SonosNet. Either way, it’d be nice to be able to move them around the house and let them use sonos’ mesh rather than the router’s wifi.

It's impossible.  Sonosnet requires stationary nodes so the configuration of the Sonosnet mesh remains stable.  Moving a wired unit even a few feet requires unplugging the unit,, triggering a mesh reconfiguration before the unit comes up. The portable units are by definition supposed to be moved on the fly, requiring real time reconfiguration, something which will kill reliability.  This is why Sonos made them WiFi only.

So, not impossible, just intentionally precluded. Probably makes sense for most users who move them. We never move ours; we just preferred the small form factor of the Roams for certain rooms.