Theres no need but for shroud normal there really shouldnt be a problem with taking more than one arcane or even not taking an arcane?I obviously hit a nerve with the amount of neg rep I've received for this comment in just the past day. Let me further explain why arcanes don't stack well in Shroud:
Part 1: Trash mobs can be easily soloable by a single arcane with Wail of the Banshee and Finger of Death (even on elite). Multiple Ice Storms don't stack on the portals.
Part 2: Fire elemental is easily soloable by an arcane, even on elite. The earth elemental is easily soloable by an arcane on normal/hard. On elite it's possible to solo, but it's more time efficient to have a melee tag team with the arcane to kill it.
Part 3: N/A.
Part 4: Devil Trash - Only one arcane is needed for disco ball or mass hold monster. Harry - Only one arcane is needed to cast Cloudkill for 20% concealment. Gnolls - One one arcane is needed to kill the 8 channelers.
Part 5: Again, just one arcane is needed for Cloudkill. Chain casting Polar Ray and keeping up Ice Storm, arcanes run out of mana before Harry is dead. It's not optimal to bring multiple of the same class that are going to run out of gas before the fight is over.
You can give me neg rep for my comments, but it's misdirected. I'm simply illustrating the fact that Turbine has failed to create content where arcanes stack well. Please direct your hate and disdain at them, not me. I'm just an observer like the rest of you.
And yes, you can complete Shroud with any composition of competent players (I've completed elite Shroud with 7 arcanes before, but they were all guildies). When it comes to pugs though, quality of player is often a crapshoot. So it's not surprising to have leaders limit classes with known stacking issues, like arcanes. If you find your class filtered out of Shroud LFMs, there's little you can do but start your own Shroud or find a static group of people to play with.