It's really not an exaggeration to say that our handwriting is a direct reflection of who we are. Illegible, poorly organized written work reflects poorly organized thoughts from a poorly organized brain, and prejudices those who have to read it against the writer. It is a mark of respect and consideration for others to make sure that writing is legible.
Children who don't have good handwriting and struggle to complete assignments on time don't do as well in school as they could. Unfortunately, there's an epidemic of bad handwriting in this country. There are several reasons for this, but mainly it's because very few people understand how to teach it anymore.
If what your child is producing is not acceptable and/or too slow, you may have to either find a handwriting specialist in your area, or tackle it yourself with the Handwriting Without Tears materials. Either way, if your child can't write legibly, it's up to you to make sure that he can.