Most bus routes here are infrequent, with buses no more often than every 30 minutes. What would count as frequent? I’m sure the majority of those who don’t use transit think every 15 minutes is frequent. But I’d bet most would change their minds after going to a bus stop and waiting 10-15 minutes (or more) for such a “frequent” bus. In the abstract 15 minutes isn’t much time, but at a bus stop it’s forever. Which is why truly frequent service is every 8 minutes or better, which doesn’t allow much time for getting annoyed with waiting.
If using schedules or trip planners, any frequency increase certainly improves the transit experience. But though we are tempted to think of 15 minute service as being frequent, it just is not good enough to be painlessly used without schedules. The difficulty with middling headways of 15-20 minutes is that while it seems like one should be able to use the service without a schedule, in practice doing so is not advisable. Even schedule-using riders may be more lax following the timetable than with avowedly infrequent service, and so more likely to just miss the bus — leaving them to wait the full 15 minutes until the next one.
Transit agencies, politicians, and transit advocates should call a spade a spade, and only refer to transit services as frequent if they can be used painlessly without a schedule by most people — and not just the transit-dependent, either.