I don’t care either way, because I can quest and solo as holy, but I’d play shadow if I could dual spec.
I don’t think it would negatively affect the game, anyone worried that their guilds would “force” them to carry 2 sets of gear and respec is either in a casual guild that doesn’t make them do the bare minimum let alone min max anything. (PS tanks already carry multiple sets of gear)
OR
They’re in a more demanding guild which already makes them do that.
At the end of the day the only group not hardcore raid logging, are doing arena which may solve the lack of arena play with less serious players, and arena players who do pve as well.
Anyone who thinks it will solve the lack of tanks for heroics is huffing copium - I’ve never played TBC (OG, private server) that didn’t end up with a hardcore tank shortage after the first 3-6 months of launch when everyone who needed rep and badges is done.