They would still be able to progress it even if they don't camp... but they will camp because it'll be done in fewer matches. It's just like asking for invasion kills in Gambit people will just start camping the invasion portal at the detriment to their team just to get the most efficient progress on their quest step or challenge. They don't have to camp the invasion portal and could still help out their team, but why bother when someone on your team might jump in the portal hindering your progress? Just like trying to camp a point then head to heavy just for a teammate or enemy to pick it up which sometimes is the only box of heavy in that entire match making you need to play an entire other match just to progress. Heavy weapon kills in normal crucible control or most other modes are fine, but with Iron Banner you come across the issues with the hunt mechanic which can make matches a steam roll and only have 1 ammo box spawn during the entire match. Ability kills, precision kills, elemental kills, special weapon kills, specific weapon kills these are all reasonable and can be done or progressed in every match you play.