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)

Bill