in the american northeast, backup knots on a figure 8 are relatively common in gyms and rare at the crag. (and when you do see them at the crag, they're a good gumby detector)
About 5-6 years ago the gyms became much more honest in their instruction for new climbers, and will now actually say the quiet part out loud, which is that a backup knot is really there to 1. ensure that new climbers leave enough of a tail on the figure 8 and 2. are a way to prevent that probably extremely conservative tail from whacking you in the balls when you take a whipper.
sport gyms absolutely do not like people who insist on tying in with a bowline.
and when you do see them at the crag, they're a good gumby detector
Huh, really? Most of the people I know who tie-in with the figure-8 use the standard overhand safety knot (or they pull the tail back through the bottom loop of the eight). Most of us have been climbing for decades. I guess old habits die hard?
I would never trust my safety to a bowline. It comes loose with a minimum of working. Only under constant load would I use it, and only then when double hitched, and probably with a finisher clove hitch.
Even my dog gets away from a bowline with a minimum of moving around the front yard.
I rarely see people tying in with a single bowline, for the reason you give. I've been using a double bowline with a stopper knot (http://www.vdiffclimbing.com/double-bowline/) for many years now (well, at least outside of the climbing gym). I've never had an issue with the knot loosening when not under load. The usual complaint I hear from people about using it to tie-in is that it is harder for your partner to visually verify. The figure-8 follow-through is very easy to visually check.
ETA: The last person I saw tie-in with a single bowline was Christian Griffith (of Verve fame) using it to tie-back in after cleaning the anchors of a warm-up in the Ruckman Cave at Rifle Mountain Park. He also didn't secure the rope whe he untied to thread the anchors, choosing instead to just step on it. Not exactly a great example of anchor safety...
Ive also always been in the the camp that this "backup" knot isn't backing up anything. If you want to finish the knot, the correct way to do it would be a Yosemite finish.
The balancing act the gyms, in particular, are trying to play is that knots should be both secure and easily verifiable on sight by gym staff, more or less entirely for liability reasons. A yosemite finish, or a bowline, are both dead-ass safe and in some cases better than a traced eight, but they're harder to glance at and know they're safe. That's totally fine at the crag, but it's a liability issue at the gym.
About 5-6 years ago the gyms became much more honest in their instruction for new climbers, and will now actually say the quiet part out loud, which is that a backup knot is really there to 1. ensure that new climbers leave enough of a tail on the figure 8 and 2. are a way to prevent that probably extremely conservative tail from whacking you in the balls when you take a whipper.
sport gyms absolutely do not like people who insist on tying in with a bowline.