I think it's not so much "tell students to improve their English", but to make the point that coherent, precise English (if that's the ambient language) is important for communication. And to give specific details in feedback on write-ups, as I do in my graduate math courses.
It is certainly true that there is a widespread belief that math (for example) is insensitive to the ambient language. Of course, in some sense, this is true. But the communication of math certainly depends on the ambient language, unless we "go completely symbolic", which (in my opinion) leads to pretty unreadable stuff. It is inefficient to not take advantage of the built-in features of the ambient language... oop, but, then, we have to be acquainted with them.
A specific claim I'd make is that "natural language" (English, for example) is more robust to typos and small errors than is very formal, terse, symbolic mathematical writing. This is both an argument in favor of not-so-formal mathematical writing, and an explanation of why somewhat-flawed English mathematical writing is still "decipherable", at least by experienced people. But, then, there is also the point that this decipherability is much more difficult when the reader is not sufficiently expert...
... so, apart from getting grades on homework, improved quality of ambient language writing improves communication. (Not surprising...)