My 2c:
For general coastal cruising I no longer carry a
sea anchor. I do carry a
drogue, but that is because I'm on a cat. On a mono, for typical short cruising hops I would probably not even carry that.
For more serious
offshore, maybe, depends upon the season and area of ocean. I've been in plenty of
offshore heavy
weather (up to Cat 1
hurricane force) and never been in a situation where I felt the sea
anchor was necessary (...although I have thought about it once or twice), but it is at least reassuring to have the option when truly offshore (offshore lots of options are good). Plenty of others who have used them successfully in offshore heavy weather.