well, my principal weapon is the FT, so I don't understand why the baelfire is not useful to me, and I want to use a DPS build (grenade) useful in Dun and WvW, so probably Rampage is better I guess .
You gonna have more dmg from the direct Damage of your flamethrower. At the same time, the Rune of Baelfire is just not that good. So in my opinion, you gonna do better overall dmg with something else.
Engineer is an emergent profession both in PvE and PvP. They are distinct profession to the GW universe ''lore-wise'', so the profession didn't receive the love other profession receive from the start. People begin to understand the strenght of the profession and build like the 100 nades begin to show these strength to the community, making this profession more and more use. The fact that a very few percentage of people choice the engineer as their first character is the main reason why we don't see that much engineer right now. This is not a bad design or unbalance from Anet, this is just how people reacted to this profession in the first time of the game.