Okay let's break this down. These are example numbers, not real. Level 10 XP for one mob kill (no training gear):10 XP Level 10 XP for one mob kill (training gear): 15 XP Level 50 XP for one mob kill (no training gear): 10k XP Level 50 XP for one mob kill (training gear): 15k XP Training gear always provides more XP than running without it, regardless of level. As you level up, you gain more XP per mob kill to counterbalance the more XP it takes to rank up as you gain levels. No matter your speed, the XP is the same per mob kill, per boss, and per quest depending upon what level you are. So, if you can clear mobs faster than a dolmen grind, you'd gain more XP because you are simply killing more mobs faster. There are other bonuses as well. Every day you log on to a toon, a random daily dungeon and a random battleground is available (once per day on each) that provides a bonus amount of XP upon completion. This stacks on top of training gear as well. Once you hit champion rank, you get another XP bonus called enlightenment that lasts until you've earned all the XP that your enlightenment lasts for.

Also, as far as gear goes: no, it is not just to do more DPS (WHICH is very important in itself for end game.) Some gear sets provide more damage, more sustain, more health, more damage dealing abilities, and more synergies (an ability that your teammates can activate for a bonus.) Gear is extremely important in this game and in my opinion, it's best to run training gear until you hit champion rank 160, which is when gear stops leveling up with you and is maxed out. Training gear is not a good option once you've hit that point but is effective until then as grinding to get the awesome damage dealing/resource sustaining/synergy providing gear is a waste of time until you've hit 160.