Needs of the many vs. needs of the one
The first time I experienced a sudden gap in the guild roster, it was in EverQuest, and it was about as painful as it gets. The holy trinity was the foundation for any grouping in game, and epic weapons made a significant difference in your chances of defeating tough content. Our guild sucked it up and endured a multi week camp in order to help our two most active Clerics get their epics completed, and once they did we felt confident we could make huge leaps in our progression. But Zordak's corpse was still smoking when one of those two Clerics told me that she was thankful for getting her epic, but she was now going to switch and play her Ranger because it was fun.
I don't remember exactly what I said, but it probably sounded like a typical Gordon Ramsay epic meltdown when the risotto's not cooked right. I vented because the way I saw it, this Cleric was being selfish in wanting to switch. It took the guild's time (and quite a bit of it, I might add) in order to complete the quest to get the epic, and because of that, I felt it wasn't up to her to decide what to play. Looking back, it was a little more complicated than that. The game made her choose between her need (to have fun playing) with the guild's need (to collectively work together in order to defeat challenging content). And because it took so long to level up a class in EQ, it was hard to fill gaps if you were missing certain classes. The timing of her announcement was less than ideal, but even if she had waited for a couple of months, it still would have been a blow to the guild, and there still would have been a conflict between her needs and the guild's needs.
The Trinity is dead, long live the Trinity
For better or worse, many MMOs are designed with a much more flexible class system, so you don't need the holy trinity in order to get things done. That makes it a bit easier on guilds because players can get creative with things like class specs and skill trees to put together an unorthodox party that still works. Maybe your best healer isn't available one night, but you're able to get three or four others spec'd out with partial healing abilities to cover things well. Or maybe you're a little light on DPS, so a couple of your "sturdier" fighters drop their mitigation in favor of increased damage. As long as you can collectively cover what's needed for the fight, you've won half the battle.
However, it takes more than just smart speccing in order to make progress. There are certain intangibles needed for certain roles. You might not need a pure tank class anymore, but your main tank still needs to have an understanding of mob placement and good communication because he's the one who is best primed to call out adjustments during the fight. Anyone can spec to cover heals, but good healers are the ones who know how to react quickly and find just the right balance between underhealing (or death) and overhealing (which can eventually mean death also). The trinity might be on its way to extinction, but the skills needed to fill those roles are still there.
The other factor is gear because someone who switches class roles often will need a new set of gear stats in order to be effective and survive challenging content. Even with a softer class system, many MMOs still have a firm delineation when it comes to who can use what types of armor and weapons. So someone who wants to play both a healing role and a tank role will need to work on completing two sets of gear, and that can sometimes put a strain on a guild's time and effort. What happens when your best tank puts in for an item that is also being eyed by your best healer, who only moonlights as a tank?
Groups are good
If you run multi-party content, like endgame raiding or PvP, it helps to have free time available so that members can run in small groups together. Grouping lets players try out new roles without too many critical eyes adding to the pressure. And by doing a little role-reversal, players can jump in with tips for each other based on their own experience with particular roles. The gear that drops should help get many of the members no matter what job they have. What you end up getting is a guild that's well-versed in many different areas, so guild leaders have many more options on any given guild event.
MMOs might have mellowed out when it comes to set classes, but there will always be specific roles that each player in the group will need to assume. Everyone might be able to tweak and spec to fill every role, but that doesn't mean everyone has what it takes to actually perform the role well. Guild leaders need to find a balance so that they have a deep enough bench but don't get mired in farming for every member in every role. Those who find a happy medium will have an easier time filling gaps and maintaining progress overall.
Do you have a guild problem that you just can't seem to resolve? Have a guild issue that you'd like to discuss? Every week, Karen Bryan takes on reader questions about guild management right here in The Guild Counsel column. She'll offer advice, give practical tips, and even provide a shoulder to lean on for those who are taking up the challenging task of running a guild.