Not an unreasonable request. Considering the written word is our only means of expression on this forum (for the most part) I can see no reason why you would not want to encourage that posts are at least legible.
Typographical errors and minor mistakes not withstanding.
Also pointing out grammatical errors that do not prevent someone from understanding a statement is just plain stupid. People make mistakes or frankly do not have the same level of language skills as the next or they type like they talk and subsequently have mistakes.
Heck I can find at least half a dozen if not more grammatical errors and punctuation mistakes in my post alone.