I'm on the side of Spinksville. The "need" for switching specs is not new to 3.1. The design of the game is simply such that you will want different amounts of tanks, healers, and DPS on various fights. If every fight took exactly 2 tanks, 7 healers, 7 melee DPS, and 9 ranged dps - encounter design would be so ridiculously limited.
For serious guilds, dual (hell, even tri- or quad-specs) have existed since the introduction of the mage portal & warlock summon combo.
Why then, do many guilds not currently have people swapping specs all the time?
Because players always have one spec they are best at.
"If my Druid can now, all in one character, walk into a raid or instance and, within seconds, either be a main healer, main tank or equitable melee or ranged DPS, whatever the Raid Leader wants at the time, AND be as good at it as the other classes are in each role, and the Rogue or Mage can only be a DPS, a DPS or a DPS?" - BBBNews Flash: No human is able to tank as well as a dedicated tank, heal as well as a dedicated healer, and DPS as well as a dedicated DPS. Its pure hubris to imagine that you can play all three rolls as well as someone that does just one, full time.
The capacity of the class does not equal the capacity of the player. You've probably met a Ret Paladin that can out DPS a rogue. That same Ret Paladin cannot out heal (in his dual spec holy set) the dedicated paladin healer, who in turn, cannot out DPS that same rogue (when dual-spec'ed Ret).
If your worried about someone else taking your spot - start learning how to play your role better. This goes for everyone - pure class or hybrid. If you can do your job better then anyone else, you are still the one getting brought to a raid. Not the person that pretends to do another job sometimes - they will still get sat out when their main spec isn't needed, on difficult content, for someone who does the needed job 100% of the time.
And as for Pure DPS classes complaining that they get nothing - how quickly have you forgotten about the "injustice" of mages having to spec out of Fire/Frost for Alar/Hydross, rogues spec out of poisons or backstab for mechanical or positionally unfriendly bosses, or Hunters/Warlocks having to spec out of pets for pet unfriendly bosses? Again, the ability to have add a strange/flavorful mechanic to a fight gives developers the ability to design a wider variety of awesome, fun fights.
Everyone benefits from this.