I think it could be a good way to get decent current discharge even from a weak battery. Basically you only need high current during a few seconds while accelerating, the rest of the time you don't need nearly as much. Weak batteries will have lots of voltage sag under load, lowering the bike's performance, but that can be compensated with supercaps in parallel with the battery which will deliver current instantly and then recharge whenever the load decreases (meaning you probably need something to limit the caps charge speed, because otherwise it will just add even more load to the battery)
So this could potentially give you more instant power while protecting the battery at the same time.
Aside from the added cost and complexity, I don't see any significant drawback.
I don't see any problem with sparks and whatnot: nowadays people use BMS, these things can ramp up the current at startup to avoid inrush so that is a problem of the past.
That's actually something I always wanted to try, just never really had the opportunity yet. Would be nice to see someone sharing some actual experiments on such system.