Arrangers Keyboards have always been designed for the home market (There just keyboards based on the easy play features that were first introduced on home organs) with probably about 99% being used in this role, however you will find many out in the wild (Usually TOTL models though) being used by professionals who want something light and compact. (Although over the last few years (Particularly in the UK & Europe) pro players have been adding a second keyboard and pedalboard to turn then back into portable modular organ, so that they can get away from being stuck with having to use boring and monotonous styles)
Home users have also started to move to the multi-keyboard line-up route, (I think Roland is the only Arranger that has not had a dedicated multi-keyboard organ designed for it) as users find it more satisfying to play the instruments, rather than playing along to a style. (I also suspect that manufactures will be adding more real time controls to give some of the flexibility of a Workstation to try and entice younger users)
As to Midi sockets, then it confuses most home users as they try and work out how to use them, whereas virtually everybody knows how to use a USB cable, and this combined with the fact that all modern OS have Midi Drivers built in, makes it a logical choice. (Even if an arranger has Midi sockets built in, the Midi features supplied are usually pretty limited)
I think the real power of arrangers is indeed the ease of use and a layout perfect for playing live, where backings, keyboardsounds are sepperated interfaces, while still being integrated..
The philosofy behind arrangers just makes them perfect for one mand bands and duo's yielding great resluts with top class sounds and clear operating structure..
I somewhat dissagre with you that home users go a 2 keyboard route these days.. tough i agree they want to play an instrument, and thats why digital piano's are extremely popular these days and arrangers are somewhat on the way back..
However all those nifty arranger features, do fit perfectly on top of a digital piano, which gives the students both a great piano to play solo on and some accompaniments for quick fun..
In the end i am convinced that Totl workstations and Totl arrangers will be replaced in the future by instruments that combine the best of a stage piano with the best of arrangers (easy opperation) that have a whole deep edditable sound engine hidden under the hood... offering 88 weighted piano keys for an instrument not weighting more then 25 pounds.. just look at the Privia PX-5S, it has weigthed keys great piano action at only 22 pounds..
I dont think people will be wanting to have multiple keyboards in their livingroom, unless its a great piece of furniture (Wersi Organ) but then, when it comes to furniture and looks, nothing beats a piano.. and thats where the ladies in the househeld of musicall kids will have an important vote..