How many networks already leak numerous unnecessary /24s to their transit providers, who accept them (not having been asked to do anything else), and contribute to table bloat?  Quite a lot of networks do this.

Sure. Even as a prophylactic measure against route hijacking if they aren't using the space for internet routed purposes (company uses a prefix internally, say for VPNs, addresses in the prefixes aren't reachable over the Internet but they announce it anyway to discourage the block being used by someone else or to ensure that wayward traffic finds a home and can be logged for correcting misconfigured VPNs).

I would hate to see all those accidental announcements suddenly appear in my routing table; or for my transit providers to have the bear the expense of dealing with them.

The probability of service-impacting accidents would definitely increase.