This graph is a simulation of mana generated by a magician in a raid situation. To compare, it shows the mana generated by a cleric in a raid situation. Once there are enough clerics in the CH chain (and everyone now uses a CH chain - current content forces it) that spell gems refresh before having to do the next heal, each additional cleric adds only their mana regen, and nothing more.
Both the magician and the cleric have all raid buffs, all AA's, and FT15. This assumes there are no mod rods available before the fight, i.e. this is only the mod rods the magician creates. The magician, when low mana, uses his own mod rods (and subtracts them from the mana he has generated). It is assumed that the magician has full time torpor.
Thus adding a magician is roughly equivalent to adding 8 clerics to the chain. If 30 rods are summoned up before the fight, and the fight lasts 10 minutes, that value increases to 12 clerics.
What if there are plenty of clerics, and they don't need to use rods? The cleric line above is roughly the same for wizards, or any other casting class. If there is any need at all for mana, it's better for the magician to summon rods than use that mana directly nuking. Even if the only class available to use that mana for nuking is a cleric, it's still far more effecient for the magician to create rods for the cleric to use for nuking mana than it is for the magician to nuke himself.
Summoning rods sucks. Gimp this please in PoP.
There is an alternative to gimping mod rods: fast fights. In a fast fight, mana regen isn't a factor. If the fight is only a couple of minutes long at max, there isn't any need to pump out mod rods during the fight. Magicians are then valuable for their nuking ability, and will have fun using it. Unfortunately, manaburn exists.