The importance of writing well can never be over-stressed. And I am not talking of code, I am talking of code comments, and even more importantly, of deliverable documents (proposal documents, technical baseline documents, user and / or administration manuals, meeting minutes, etc.) as well as formal communications.
We're talking of "proper" English (or whatever language you have to write on - I am not a native English speaker but I have had for several years to work in English). If you don't do it, your customers might think you're an amateur, unprofessional person and might run away from your... software (which is a curious thing).
There are good resources on how to write better, you might be interested in researching and subscribing to them (if applicable):
- One example of a quick recap you can find in the WWW:
Use of capital letters:
http://wmgroup.cmail2.com/t/ViewEmail/i/941747E3A36E00D2/5A0433250211844FC5EC08CADFFC107B
Doubled consonants:
https://en.wikipedia.org/wiki/American_and_British_English_spelling_differences#Doubled_consonants
- More and in "small doses" like the above (to learn a little bit at a time, as it typically works better): https://twitter.com/thewmgroup/status/695639693795446787?lang=bg
@2016-11-04: Added consonants topic
