I've told my guide that I prefer communicating via email. He responded by saying that was ok with him, but he needed me to know that he only checks his email at the beginning and end of his work days, and not when he's out of the office (for example, on vacation)--he does check voice mail more frequently, and when he's out, though. So, I don't expect replies til the end of the day as I know he doesn't check his email during his work day. And if I don't hear back the next day, I know he's out of the office. THen I call if it's something I need quickly, or just wait to hear from him if it's not time sensitive.
Just the other day I told him I want to add on some BCV points, and asked him to put me on the BCV waiting list, and gave him my credit card number (he needed that to process it)...all thru email and all went smoothly. He did pull up his email for the 3-4 back-and-forth emails this request required--took about an hour in total...he's really very accommodating. I like our guide a whole lot--if you're looking for a guide and want to know more about mine, feel free to PM me...
So yes, guides can use email, altho it may be that they wait for their clients to tell them that's their preferred method of communication.