I'm confused. In what way would the optical rotaries be more versatile than the mechanicals? It seems to me that if you already have a spinner the optical rotaries become redundant. Too late if I'm wrong. I ordered my druin interface three days ago. 
I think Rampy summed it up ok. One reason I can think of is I want a more standard joystick than compared to the Ikari type when playing 2 joystick games like Robotron and Smash TV. It is weird with 1 Happs Super in the left hand and an Ikari in the right hand. I guess a Happs mechanical rotary would solve this too.
Like Rampy said, in a pinch, I could use the 2 optical rotarys for other types of games and not have the mechanical clicking of the Ikari joys. Yes the Ikari joys are more authentic than the opticals. I also rather have a real Frontline rotary control when playing that (I actually have 2) but I can't have all of them on there

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The optical rotarys can also substitute for a second spinner when playing a two player spinner game. Of course this is just my opinion. I think others here have agreed that the opticals are probably a little more versatile. Also only 1 interface to worry about. Just my .02.