If you want to keep the mage, then keep the mage.
There's nothing wrong with playing what you enjoy.

I personally really like having at least one shammy in any caster type group.
280 party spell power, casting haste, health or mana regen and tremor totem.
On a mail wearing, shield using caster.
Bloodlust/Heroism is nice, they can dps and heal well enough.
They can rez others and self rez too.

That's not to say your team will not work, or that you cannot clear every heroic with the current group.
A Pally is a pretty good tank, and easy to macro (96969) as a boxed tank.
A Priest is a very good healer. Discipline has a lot of mitigation and Holy has great AoE heals.
The warlock can spec however you like, and with click castsequences will dps well.

The mage mana issue can be mitigated somewhat by having one go Frost, which gets replenishment.
Frost isn't a waste, in that you'll snare things at range and possibly freeze them in place.
Blizzard is an excellent AoE, and the pally is excellent at holding AoE threat.

I'd probably go Arcane on the second mage, just because its a lot easier then Fire.
With Fire, you need to watch for specific procs and then cast a given spell.
Then again, with an addon like MSBT set to display procs etc, you can macro whatever too.

Outside of high level arena, anything can be made to work.
An all caster group (aside from the tank), is especially easy.
Although interact-with-target, has made melee much easier too.
If a given composition is fun, go with it.