I would put a reasonable floor on short TTLs (5 minutes?), but yes, it's nice in an emergency to be able to send everyone someplace new. Sucks if you're down, hacked, etc, and can't do anything about an existing long TTL other than wait it out.

Edit: Worth noting there's lots of software that seems to only resolve hostnames at first connection, then hangs onto it forever. Lots of java internals for example, unless you poke in specific configuration.