Depends on the regen setting and/or capability of the controller.
On my CrazyBike2, the braking even using both motors at once is minimal; two typical 12FET controllers.
On SB Cruiser using 12FET that does active EABS braking, rather than just regen, it's really hard even though only the right motor is doing it. But this actually *uses* power to brake, rather than regenerating it back into the battery.
None of the controllers presently on either vehicle are programmable to change their regen settings, but some are.
As for pedalling and gears--that entirely depends on how you ride and what the front gearing is on the bike.
I use SS gearing on the rear of CB2 because I can't really pedal enough to do any real work moving it. In an emergency, if everything dies on there, I could pedal it a few yards at a time, stop and wait a (long) while, then continue, but it's easier on me to walk it home.
On the trike, it has SS gearing because it was too complicated to set it up for more than that when building it originally, as we were kind of in a hurry.
Also, the way the rear frame is made it can only hold a singlespeed-setup wheel. I have a plan to add more gears to it for emergency pedal-home and possibly a middrive, but none of what I will do applies to a bike.
For someone interested in pedalling along with the motor or unpowered, there's not much reason to just use singlespeed other than simplicity of setup (no derailer, no shifter or cable, shorter chain, lighter weight by a little bit, etc).
For someone not interested in pedalling, there's not much reason *not* to go SS.
If there's no law requiring operable pedals, you could just leave the whole freewheel and chain and pedals off entirely, and just put pegs on the BB, if you really want to save weight.
Or leave the pedals so you can ghost pedal for whatever reason you might have for doing so, but remove the rest for simplicity and weight.
Personally, I'd rather have the pedals and chain all operable just so I can get back home without walking it, if I absolutely had to, though at this point on a heavy cargo bike I couldn't do it unless it was a trike or had outrigger wheels to hold it upright so I could go at really really slow speed and gear it way way down to not have to put so much torque into the pedals directly (which really hurts).
