Silly question....are people afraid of using flash for indoor events LOL!!!
If I wanted to document an event with available light only I'll bring a prime. If the venue is dark I'll use flash...This is where I make a "call" and determine if I'm going to use f/2.8 zooms for convenience supplemented with flash or stick with 2 bodies and 2 primes.
Some situations where I do not want to disturb neighbouring tables at a restaurant I'll shoot primes only .
For events I couldn't image buying a 16-35L f/4 IS or this is where I'll have to pull out the speedlight flash sooner than I'd want to......