Pain in the arse if you stall at the lights too. Gotta shift back into neutral before you can re-start.
Also makes the bike practically unrideable if you break a clutch cable (need to start in gear to get off the line). I'd rather not re-wire the bike just to get home when a clutch cable breaks, which has happened to me on several occasions over the years.
I've never blown up a starter motor, and quite frankly I can't see how locking out the power supply to the starter when the bike is in gear achieves anything. The sprag clutch prevents the starter motor being driven by the engine. If you hit the starter button with the engine running, so what? The starter motor won't spin fast enough to catch up to engine speed and engage the sprag clutch.
On those rare occasions when the sprag has seized and destroyed the starter motor, I very much doubt it was caused by hitting the starter button whilst riding the bike.
The clutch interlock for the starter on my 'modern' bike annoys the crap out of me too. The starter will only work if you pull the clutch lever - even in neutral, which pisses me off.
As you can guess, I'm not a fan of idiot-proof interlocks. I don't like machines that do the thinking for me. I already know that I shouldn't press the starter while cruising along the highway. I don't need a set of relays to tell me that.
I don't like automatic transmissions for the same reason. Don't get me started on automatic windscreen wipers, headlights, etc. It's the thin edge of the driverless vehicle wedge.
End of rant. Sorry.