You can get away with a lot in a five-man instance: shadow priest healing? No problem. Four hunters? Trap away! Does that flexibility extend to raids? I'm guessing not. My guild is just starting to think seriously about raiding and this is one of the issues we will inevitably have to tackle.
There are really two questions here: firstly, do you have a balance of classes in your raid pool, and secondly, are they specced for the roles you need in your raid?
Having never raided before, I can speak with the complete conviction of a novice and say I believe there is a role for every class in a raid. And probably for every possible spec of those classes. That suggests to me that a good raid group would have a balanced mix of classes. But it's virutally impossible to find that balance on a server where hunters outnumber all other classes and good tanks are rarer than gold dust. So immediately, you might see an important gap. Buffs like Gift of the Wild, blessings, and certain crowd control can make your job as a raid group that much more achievable.
And what roles are those classes going to play? We all have to get to 70 somehow, and most of us choose the high-damage route. So you end up with a guild of retribution paladins, enhancement shaman and feral druids. And, er, shadow priests. And there is probably a role for all of these players in a raid. Just not all at the same time. This is a fundamental flaw in the game: it encourages all players, of all classes, to get good at killing things on their own in order to level. But in doing so, you learn one style of play at the expense of all others. I've grown rather fond of melting faces and don't much fancy the prospect of going holy. Not to mention the fact that I haven't done much healing and would pretty much have to learn my class all over again. I imagine the same difficulties for all of the multi-role classes.
There's no point in forcing players to fulfil a predetermined raid role — they will either suck at it or grow bored and leave. But unless you have a good balance of classes and roles, you'll at least have to ask players to consider it. The decision could mean the difference between them making the raid and sitting it out.
D&D 5E House Rules: Randomness
13 hours ago