Right you are UD. On Yamaha arrangers (Tyros, PSR2100, 9000pro, etc), I play in 'split' mode because I find Yamaha's 'full keyboard' mode chord recognition unacceptable.

For some reason (in ‘full keyboard’ mode), when you play a chord inversion with the left hand and then play (with your right hand), ANY note located within one octave above any of the notes being played with the left hand (other than the chord’s root note), the chord is recognized as an ‘on bass’ chord. This means that if you play a 1st inversion C7 chord (E2-G2-Bb2-C3) with your left hand, and play (in your right hand), the note E3 or G3, the chord will be recognized as an ‘on bass’ chord C7/E and the auto accompanmint bass part will play an E note repeatedly. This in itself isn't neccesarily bad, but it sounds jarring & inappropriate when playing a melody or soloing in the right hand, as the chord recognition (and associated bass pattern) abruptly shifts back and forth between C7 (C bass line pattern) and the C7/E ‘on bass’ note (single E note) as you play (with your right hand) the scale: C3 – D3- E-3 – F3 – G3, A3 – Bb3 – C4.

'Full keyboard' mode on Technics brand KN arranger keyboards, on the other hand, works well, allowing you to solo (play) any note contained in the 'chord scale' of the chord played with your left hand, without the auto accomp chord recognition abruptly switching to an 'on Bass' type chord.

Here's a link to an earlier thread on this topic:
http://www.synthzone.com/ubbs/Forum37/HTML/002674.html

Scott