The "no manpower for you!" events attached to professionalism are, in general, rather annoying, but that one is probably the worst. Frankly, I'd rather have comets any day.
It's not really free professionalism, as running out of/low on manpower means burning prof via either mercs or slackening. Getting that manpower back via slacken will cost 2x, maybe 4x as much prof as you gain. Also, if you're already at 100% prof, it's just a pure kick in the teeth either way - lose manpower for no gain, or lose professionalism (thus slower drilling and more expensive generals).
All in all, it makes trying to play a manpower-reliant game even harder than it already would be. That may be a good thing or a bad thing, based on your perspective, but it's certainly one more argument in favor of just using prof as a manpower generator, vs. actually sitting on it maxed out.