I run in to the same problem. My customers would rather that I would do their work and nobody else. In fact some call me direct instead of going through my dispatcher and they will wait for me. However, to keep turn around time to a minimum most companies need to grab the "nearest" tech who is available to take care of the call.
There also is the problem of the tech buying their own van and going it solo, which happens and companies lose work. It really is a catch 22 for many companies, satisfy customers by sending the same techs with the possibility of losing customers if the tech goes on their own, or send whoever is free and possibly lose customers because you are probably sending a second rate tech.
I find that the smaller the company the more likely it is that the same tech will service the same account. It's when the companies grow to a huge size (1,000's of customers) that you find different techs going to different customers all the time.