I am not sure I totally agree with this. I think it really all depends on how you market it. If your site and all your adds are aimed at the working family that does not want to take time off of work to either call you or come into your shop or wait for you to show up and pickup the PC. Then you will get those types of people calling. I would love to be able to have to cable guy come over to fix problems after hours and not anywhere between noon and 5 PM.
Will you miss some people? Sure you will. Not everyone has the luxury of having a retail shop or the funds to quit their job and do just this from scratch. Do what you can to get the customers you are looking for now. Get good word of mouth going and let it grow. It will be a little slower than doing it full time but it will grow if you do it right.
I must say that this is all theory and may be totally of base as its my plan as well. I also plan on having my wife answer the phone just to schedule appt and get a basic idea of the job and then I will call them back.
My very last client left a v mail for me and I did the work. It does happen.
I'm fortunate enough to live in an area where options are very limited for computer repair, and I have heard negative comments about the 2 other shops in town as well.
I'm not worried about the business exploding at first as you mention, and am willing to allow it to slowly grow.
You may very well be right, and perhaps people won't leave voice mails... and Ill change how I operate things if that shows to be true.