Well, it's a fair point. After all, language is designed to get your point across in a manner that the other person (or, in this case, people) understand. Someone with the best command of the language still needs worthwhile things to say, otherwise they get annoying very, very quickly.
On the other hand, though, text-based games are just that - all text. Despite the fact that people have taken to chatting over the Internet in the same way that they would in real life - that is, casual, "fingers before mind" kind of stuff - we lack a lot of comprehension aids. We can't see facial expressions or gestures. We can't sense tones or voice effects. It takes a lot of effort to get a sense of "feeling" across.
This has two problems: firstly, roleplay is a lot about feeling. It is about evoking a scene in someone's imagination, and creating one in your own. The trick is that for things to make sense and play out seamlessly, this scene has to translate nearly perfectly. If two people are imagining totally different things, there isn't much hope for any kind of happy ending. In addition, there is a more general communication breakdown: if people can't understand what you're writing, they just can't communicate with you. They can't tell what you're saying, what you're asking for help about, what response you want to get from them. It isn't that they're trying to be rude or unhelpful, they just don't know what it is you are trying to get across. Part of this is, of course, that the sender has to realise that they are not the centre of the universe and people honestly don't care enough, nor should they, to spend ages trying to decipher the message. If they don't get it first time, it may as well never have been said.
Following on from that is the second problem: it just shows a lack of respect. MUD communities are a joint effort, like a novel being written by different authors, where each author has no editing power over what the others put in. Once it's said, it can't be undone by someone else. So if one person is spending two seconds on every sentence, the novel is going to look awful no matter how good the other authors are. And then what happens? Of course the other authors are going to think "well, why should I stay here and get dragged down by so-and-so who either doesn't know what they're doing and doesn't care?" - they go and find another book to contribute to. One bad apple can ruin the whole cartload for everyone.
So what do I suggest? Firstly, I like the above analogy a lot. Think about a MUD as being a co-written novel, a very flexible one that has no defined beginning or end. A work in progress. For all you know, someone is logging the goings-on for posterity, to look at later for inspiration or simply for enjoyment. Whatever the case, remember that a novel is a very fragile thing, and one writer can easily ruin the whole piece. If you feel that your writing could be better, then this is an excellent place to learn - surrounded by others who love writing, and want to help all the writers improve, so that the whole work improves. Just remember that in this medium, effort is vitally important - and it is just as important to show others that you are holding your end up in that regard.
"Bad spelling can be lethal. For example, the greedy Seriph of Al-Ybi was once cursed by a badly-educated deity and for some days everything he touched turned to Glod, which happened to be the name of a small dwarf from a mountain community hundreds of miles away, who found himself magically dragged to the kingdom and relentlessly duplicated. Some two thousand Glods later the spell wore off. These days, the people of Al-Ybi are renowned for being unusually short and bad-tempered."
-Terry Pratchett, "Witches Abroad"