It's not useful.

Users sometimes click on the button multiple times.

If a connetion is slow to get established, this would cause it to
be pushed twice, the first time before it is complete.  Thanks to
Roman Koval who noticed the issue.

Apparently, it is possible for an ICE candidate to be generated
before onnegotiationneeded triggers.  Avoid losing a candidate
in that case.

Probably not necessary, but I'm supersticious.