No, I think @s3_gunzel is dead wrong. Well, he's sort of wrong. For one thing, it depends on experience. Niche websites simply aren't going to get top-of-the-line remarks from the inexperienced.
You mean they're gone once the money stops? Of course, that's why a long-term investment is needed. But I can understand why most forums can't afford it.
That's usually not the case, material wise, but sometimes there are occasional grammar and paragraph spacing mistakes. Nonetheless, some forum owners are more informal vs some more so.
I would say that if you are able to pay for forum posts it is a good idea. There are a lot of people out there who offer great services that will get you high-quality content on your forum which is what you should be looking for if you want your forum to reach a high ranking on search engines. It's always wise to look out for people who are good at their work and have work to show you before you buy.