Some people can edge with a trimmer, some can't. I know for a fact that I can edge with one, many time looks better than some edging with a dedicated edger. Of course I have done this for many years, and was once not very good nor efficient at all with it. You probably don't want to expect employees to be able to efficiently and neatly edge with a trimmer. My one client was so amazed I had been using(when I was the one on the property), a trimmer to edge, he said "wow you have a steady hand", and he is a very picky ex-marine(extra cool guy). He had stated at the beginning that he wanted a blade used for edging, so I told him that I would always have an employee use what he wanted, and let him know that I wouldn't, and explained why...he was fine with that as long as it looked above par. He has been more than pleased for the past two seasons. To reiterate, I wouldn't expect an employee to be able to edge as well as I can with a trimmer, but it would be a plus to have as a skill in their back pocket.