Although you have defended your stance quite well there are several major problems that stem from mercing.
1. Players get gear from content they are unable to clear. This in turn makes them more geared which gets them into even higher tiers of content which they not surprisingly will not have the skill to progress through. It's a huge headache to run groups where you get player like this that are geared enough to participate only to find they are terrible at their job/class.
2. Depending on the type of mercing it can create easy revenue streams for RMT.
3. It takes away from the sense of accomplishment players get from clearing content when they know any old Joe they meet with the same gear could have very well just payed his way to getting it.
4. Trade skills are meant to be diversified, players are not meant to master master all trade skills and the economy is designed that players make what they can and trade for anything else. Dungeons were not designed as part of this process, players are meant to progress through dungeon tiers naturally so as to avoid the problems of point 1.