When string trimming, I don't want to spend alot of time trimming 500 linear feet of driveway edging. Our accounts have sprinkler heads along the driveways, low voltage lighting, pet containment systems or outright small fences. Edgers are not an option to the maintenance.
I agree that less damage is sustained with a single line on fenceposts, fences,etc.
If flossing around poles and retainer walls is an issue, then slow down on the throttle with the commercial trimmers........it will trim the same way as a single line...!!!!
Some bermuda grass lawns will grow like a hellian, and a single line trimmer takes for heaven sakes--ever!! No offense here..........we bid lawns by the time it takes and the quality of the work done.
I am sure that you can perform just as a quality cut as someone with dual string heads......hell I can do it too!! I have been running a weed-eater for 19 years. Single and dual.......3-tier......bladed........and back in the early 80's I designed a spinning flat disc with piano wire leads.....similar to the design of the meg-mo's for ZTR's. My invention was way too dangerous to hit the mainstream market. It was good for grasses only and not saplings or woody stemmed weeds. The noise was weird and the balance was unpredictable.
If the velocity isn't what it should be, then shorten the line, or fill the head with .80 string. The book will recommend the appropriate line to use for the unit, but you can go smaller if needed to increase speed and production. You will probably have to replace more line and bump more. The biggest majority of spent line is trimming too close to the concrete or pavement. The line will spend on its own, and we often soak the line in water to keep it from drying out and breaking down from UV light. Oxydation will destroy the polymer and make it weaker .........!
The reason we take off the guards, is so we can see under trees better and it allows me to run like the wind while trimming. I have eyes for trimming and know how long the string is at all times by the way it cuts. It took some time to get used to the missing part but it is an integral part of our speed.
If all of this is garble to you, then using a single line trimmer is your choice and that is human nature. But for us that do this line of work 6 days a week and 10-12 hours a day; there isn't anyone going to tell me different on my line trimmer and its production. I have used everything the market has out there. It all has its place......!!! It is my opinion of each product and it only matters to the user.!!!