I am left-handed. I always thought the SeqV4 layout alright for me. On the other hand (no pun intended), the other machines I used the most are MPCs, and they (except for the 2000/2000XL) have the Menu section at the right, and that actually felt alright too.
I must stress though that I am not really a keyboard player, I only ever write drums in real time, so the switching-while-playing argument doesn't really apply. I do agree that left-handed people are just so used to doing things with their right hand, not being given the choice (computer mouse being an obvious example), that they are more comfortable to do so. EG I hold my mobile phone in my right and write text messages with my right thumb.
At home I have the sequencer on the left and the mixer on the right, and that's the configuration i always default to. I think it's because like that, I can use my better coordinated hand to do stuff on the sequencer, like using the thumb to hold the menu key and index or ring finger to switch to a different mode or mute/unmute stuff, while adjusting faders/sends on the mixer.
Now you really got me thinking if the example layout above wouldn't be better for me!