I would argue that everyone should be able to write well. Writing in an effective and clear manner is invaluable in the workforce. In most jobs you'll need to make presentations or write reports. If one's report or presentation is well constructed, clear and as engaging as possible it will save both you and the reader time. If it is poorly constructed it will take the reader longer to find the information they need. If it is not clear they will spend unnecessary time trying to figure out what you meant, and potentially need to come to you for clarifications which wastes both of your time. Finally if it is not engaging the reader will be less likely to read it thoroughly and try to get through it as quickly as possible which could cause them to not absorb the information you're trying to present.
While everyone should have good writing skills it is especially important for us geeks. Take doc strings. If yours is poorly written then the client will have trouble understanding what your methods and functions actually do. Secondly, I find good writing skills and good oral communication skills go pretty much hand in hand and a good geek needs be able to explain their theories and work effectively in both ways in order to be successful. Its no good being brilliant if no one understands what you did and can't use whatever you built.

