I think that's an argument to enable it till at least beta1. Let's change the default, and add an item to the open items list to reconsider then.
+1 during the beta, +0.95 for default thereafter.
I think that most databases in the past have defaulted to single-core unless otherwise stated because machines that had multiple cores were uncommon, and the query that could intelligently use parallel was even more uncommon. So for them, the name of the game was "plan stability".
Machines are architected to be multicore now, and that will be the norm going forward, as will larger workloads that can easily overwhelm a single CPU. So I think Postgres should just enable parallel out of the box.
Having said that, it seems like the sort of thing I'd want set-able on a per-user basis.
ALTER ROLE overly_needy_web_client SET max_parallel_degree = 1;
ALTER ROLE moar_powarrr SET max_parallel_degree = 32;
And of course this is my chance to re-ask that we not block the possibility of one day being able to set this value relative to the number of cores available on the machine, i.e. this user can have a parallel degree 2x the number of CPUs, this one can only have 0.25x as many CPUs. It would be nice to have our configurations adapt with the hardware.