Speakers: OnBoard Mixers and Input Connectivity

Hello, its me again with a question -

I realized that some speakers such as the Yamaha DXR series, JBL EONs and QSC K12s have on-board 3-channel mixers, often with versatile input connectivity at the back. But then other slightly higher end models such as the JBL PRX and Yamaha DSR series do not have such mixers and often tend to have either an XLR or 1/4inch connectors alone.

-- Why is this the case?
-- Does so many input connectivity options complicate things and defeat the KISS (keep it simple) principle?
-- if for example I want to connect an mp3 player to a speaker such as JBL series, is it possible to do so using an adapter?

I would be very grateful if I could get some insight into these as it would make my decision between the Yamaha DXR, JBL EONs and JBL PRX much much easier. I like the PRXs but I am being swayed to either the Yamaha DXR or the JBL EONs purely because the on-board mixer and versatility.

Thanks - Oyiwaa