Then they obviously didn't belong in that guild, if the guild was unwilling to accept the role that a player applied to do. So there's nothing wrong with them apping out, whichever class they play.
If what people are asked to do includes rerolling mains for a new role, then how is restricting roles on classes actually going to solve this person's problem?
Moreover, how is adding new roles to classes going to make things worse? If anything, it would make things better, because now instead of being forced to choose between rerolling mains for a new role and having to leave and find a new guild, he now has a third option: play the new role while still being able to stay on his current main.
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I have heard some arguments about whether lock tanking is actually good for the game, and I've actually considered a lot of them and changed some of my viewpoints on them. It saddens me, then, that the fallback here seems this flimsy argument based on players who "don't want to be forced to tank" and are afraid that adding more ability to a class means they are "forced to become the new main tank."
There are two kinds of people who like to only play DPS: Those who are willing to switch to tank/heals for a night maybe, but don't want that to be their main role, and those who refuse to tank/heal whatsoever point-blank.
For the former, they apply based on their role, and will understand if they have to switch for a night. The guild understands that they are willing to do so but will find a new guild if they are forced to permanently switch roles.
For the latter, I'm going to be frank. If you never tank/heal whatsoever, you probably don't have the bars for it, you probably don't have the UI/addons for it, and you probably don't know how to tank/heal (as in what to look for and stuff). The guild probably knows this, and even if its "this druid switches to heal/tank or we don't raid," there's no point in waiting for 30+ min for said person to unwillingly set up bars, download addons, and learn to heal/tank for a raid that would fall apart within an hour anyway.
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Also this isn't specific to warlock tanking.
Aversion to being "forced" to use an optional spec isn't a good reason for not including said spec. Being afraid that as a paladin, you'll be "forced" to ranged DPS is a stupid reason not to implement a holy DPS spec. Being afraid that as a rogue, you'll be "forced" to forgo your three melee specs for one ranged spec is a stupid reason not to implement a ranged rogue spec.
Hell, being afraid to play melee is a stupid reason not to play an elemental/restoration shaman. Being afraid to heal is a stupid reason for a MDPS player to forgo enhancement.
Even for the specs I mentioned above, there might be good reasons not to put the 4th specs in (the main one I can think of is "not needed" or "won't make the game better). But fear of being forced to play an optional spec is definitely not one of them.