Not as much as you may think. It's quite possible to select text using the keyboard if you have a good screen reader like JAWS or NVDA, and sighted people who don't use mice also normally have access to keyboard navigation controls in their browser. (In Firefox or Internet Explorer, for example, try pressing F7 to turn Caret Browsing on. This might work in other browsers too, but I haven't tested it.)

This isn't to say it's a perfect method, and DW certainly shouldn't be relying solely on that, because DW can't (and shouldn't) dictate what browser, assistive tools, etc. their users use, so thank you for bringing it up. Suggestions is all about brainstorming ideas, and this is exactly the sort of thing that makes DW better. :D