Main reason for magicka classes (bar magicka sorcerer): if you can't kite, you are dead. A much easier access to mobility makes stamina a favorite in this regard. Stamina Warden due to access to better overall tankiness and survivability than its "brawler" counterparts. Stam NB for general group avoidance and initiative (once ZoS fixes the numerous bugs with them)

Ok, that was a lazy list, but there are way too many factors, and different groups will be able to tackle different objectives in different ways. You can have a stamblade duo killing every single member of a group, 1 by 1, no matter how big the group is, you can have an AoE heavy group (multiple stamina wardens with timed Sub. Assault / DBoS, a destro ult combined with an AoE stun or root...) that will be able to burst down multiple targets, you have a near unkillable magplar / stamden or stamden / DK duo which can distract a pretty large group for a while... And pretty much all classes have something to do in it. Some combos work much better than others, but all classes kinda have a role here.

Most of it is shamelessly stolen from a bunch of vids taken directly from Zerg Squad, but I think the descriptions are pretty much self-explanatory...

Current highly organized groups rely on a stamina build spamming Retreating maneuvers (to make sure the whole group stays on the move,
as you would otherwise die very quickly - unless you are in an entrenched position), Purge spammers (to get rid of DoTs and possible debuffs, especially enemy sieges), healers and magblade as damage dealers. Stamina sorcerers fit the bill perfectly when it comes to sustain Rapids spamming, as they both get 20% stamina recovery on top of Dark Deal (or w/e is the morph for converting mag to stam). Negates are a very efficient tool to briefly shut down enemy healers, templars in large part due to BoL, Wardens for access to multiple AoE healing abilities and group buffs (shroomz OP). Magblades really crush any competition when it comes to damage dealing, with an access to a gap closer that isn't stopped by roots with a strong snare attached, a large PBAoE that also heals, as well as high ultimate generation in a world where Destro ults make or break a lot of fights.

Praise be #ZosBalance. Seriously, I can pick any spec, build a bit into sustain, sit there and heal or shield up and my opponent won't be able to kill me, unless the other one decides to run a stupidly niche build. At least in CP campaigns,
non-CP is less of an issue.