I think the risk of road rash in falling off at local street speed is small (see note below). Small enough that it may be acceptable to ride around with minimal gear. The people who get seriously injured at local street speeds have impact injuries or head injuries, not road rash.
Comparing it to a bicycle though, bicycles may hit 30mph at some point, but they are a lot slower overall. A motorcycle spends all it's time at 30mph on local streets. It is still higher risk than a bicycle.
(note): I used to live in Vietnam; I've seen plenty of people on scooters in shorts and flip-flops fall off at slow speeds.