With 12-position rotaries you can play the Ikari series and a lot of other games that used SNK's LS-30 joysticks.. with the analogs, -- unless u_rebel can correct me with info about support in analogmame -- you're limited to the few games that used analog rotaries like Caliber 50 and TD Fever 1-2. Wild Western, Tin Star, and Front Line use an entirely different 8-way 'rotary joy' and won't work with either -- once again, analogmame support may have been hacked, I dunno.
Mame uses "DIAL" type inputs for all 12-way and analog rotaries; ie mame likes spinners for 12-way rotary and analog rotary games. So analog rotaries are better supported by mame than 12-way rotaries.
However, MameAnalog+ 0.71.2 will have improved support for druid's and mk64's 12-way rotary interfaces. Hopefully 0.71.2 will be up tonight, but it's looking more & more like tomorrow.

The user will be able to choose between using the mame standard spinner input or druid/mk64 interface input.
Analog+ has no improved suport for 8-way rotaries. For mame & analog+, the easiest thing for this type rotary is a second 8-way stick with a trigger or thumb button.
General rotary (hardware) stuff:
Optical (aka analog) rotaries hook up to stuff like mouse hacks, optipac, and oscar's mouse hack. This is the easiest route to go, as the mouse is very PC friendly, and mame is setup this way.
12-way rotaries hook up easiest to the mk64 encoder or druid's interface. These interfaces work okay with mame, but MC-Escher's changes (to be included in analog+) are better fit for these interfaces than current mame. You
could hook one to a ipac-4 and take up 12 buttons and/or joystick direction slots, but to use this setup you'd also need to edit mame's source. One of these days, this edit
might also be included in analog+, but not yet.