Sure, languages are about communication. As long as the other person understands what you are trying to get across, you're probably fine.

However, orthography does serve an important part in getting your point across. German for example heavily relies on capitalization (compared to e.g. English) to help the reader parse the sentence structure. The Umlauts serve a similar purpose to make it easier to differentiate words.

For instant messaging this probably does not matter too much, but what about books, technical reports, etc.? What's to gain from not using "proper" orthography in them, which might give the reader more hints about the sentence structure and hence its meaning?

Schöne Grüße ;)