In deference to the fashion among you kids for listicles and stuff, here is a listicle and stuff. No cat gifs.
1) You want to win the argument. You have to bring down someone's defences so they can hear you. If you start with moral bombast, you probably won't do that. (I stress, 'probably'. It's not totally unimaginable that if you lay into someone viciously enough, they will be compelled to pause and reflect. But it's far more likely that they'll write you off as an asshole and maybe even disengage politically.)
2) You might be wrong. You might be trying to resolve a more-or-less nuanced political disagreement in your favour with a moral brickbat. And what for? Hardly to convince anyone, or build anything. It is, at best, grandstanding and self-aggrandisement. At worst, libidinised bullying.
3) It has a boomerang effect. You launch into someone with full bared teeth one day, you can be sure that a swarm of similar moral piranhas will engulf you in the near future.
4) It makes you look like the dick. Even though you may be in the right, and even though you may be actually legitimately aggrieved by something, you can still make yourself look like a complete dickhead. This also applies to anti-moralism moralism by the way.
5). I fucking hate listicles. Piss off.