I think it would discourage clients from using that form of communication for reaching out. The thing with emails is, T's can choose to respond or not, and how much they want to respond. If they find they are getting overwhelmed with emails, they could easily just reply back with "I've read your email and we can talk about it in session." That sentance took me all of 10 seconds to type.

My T allows me to email as much as I want, but I don't abuse it either...during a particularly tough week, I might send 2 or 3 emails, but there have been months where I didn't send any. One time, she didn't respond to a very long email I sent and I told her that it really bothered me that she didn't respond. So, she agreed that if she can't take the time to really respond, she would just give me a quick - got it, we'll talk later - reply.

However, my T does charge for phone calls longer than 10 minutes. I can't recall what she charges, but I can understand that, since once you pass that 10 minute mark, you're starting to eat in to her time, and really having a phone session rather than a quick coaching call.