Well for one I don't replace my Stihl Trimmers every two years or anything else Stihl for that matter. I've been running the same Stihl Trimmers for the last five years and they run as well now as when I purchased them. The theory that you talk about has nothing to do with the cost of a Trimmer. The reason cost comes into the equation is because your buying a Trimmer that has a larger engine and has the power to do jobs in less time than the typical homeowner unit and that can take the abuse that the homeowner unit cant take without constant care and attention. The weed eater trimmer is a decent unit if used by the homeowner for light use once a week, it should last a person a long while but if I have to climb down into a drainage ditch or trim around retention ponds where grass is a foot or taller, places where a mower can't go I'll take my Stihl 110R with 0.105 line and do it three times faster than you will with a weed eater feather light.
The theory i use when looking at a job is where I can make the most money in the least amount of time and I use the same theory when it comes to equipment, what equipment is going to give me the biggest advantage on the job, the answer for me has always been Stihl.