It's just the maths here. Let's say that 1 Wizard does 1M DPS, so 2 of them do 2 MDPS (which is probably not true because some buffs won't stack properly)... but a WD can increase damage more than 150%... so 1M+150% = 2.5M which is more than 2M with 2 Wizards and no WD... Plus all the crowd controls that you need to survive past GR75, etc.