I have a kronos 88 under my pa4x..

Because :

It has 88 keys and functions as a great stage piano, i have 5 different brands grandpiano’s on it
It has increible organ and synth engines
It works perfecly as a masterkeyboard comtrolling vst’s (as well as chords for the pa4x)
It comes with an incredible arpeggiator named Karma
It allows me to have more then 4 sounds assigned to my keys

And many many more reasons..
An arranger keys, has a huge amount of limitations compared to the kronos.

Sadly getting results on the kronos takes much longer then on the pa4x where it comes to sound edditing/design