I've only seen Quaffy play it, and it seems to work fine. With IWB hitting twice for 60k, he one shots anyone under 110k HP easily (assuming no ridiculous Mdef, which is about 90% of the people we fight). But surely, its would be better to play than a P/S, since lightning, fireball and Silence (Quaffy's new favourite skill, always a nice lol moment hearing him go "wow, this is awesome!!!" when he goes around and tries to silence everyone

) is better than shot and Vamps arrows (which is basically useless).
And with my limitd experiences of playing a P/M for PvE using mage gear, its not that bad either. Lightning allows you to have a pretty much Mage playstyle - root the enemy, then nuke wih IWB and RT from afar. Or use Lightning and freeze as CC if you accidently pull more mobs then you can handle, then proceed to take them out one at a time.
Although a TP starved mage is not all that terrible either, since mage skills scale linearly, you won't notice much of an imporvement after 55 anyway, and on a level 75 mage, getting skills to level 55 should not be a big issue. I recently leveled my T-storm from level 55 to level 65, and it now hits the tiles in Coarse of Terror for 96k instead of 94k. In siege and instances, that difference is pretty much non-existant.
First, remember that I have Mage leveled to 58 on my toon. It's nowhere close to *your* Mage's level--and I know you haven't updated your sig. (I have faced your guild in Siege fairly recently and I know your Mage class is at or near the level cap.)
Second, even at level 58, my Mage is starved for TP. I used up virtually *every* TP orb (100, 1K, 10K , and 100K orbs) I had, saving a few 100K orbs to put into IWB. on the Priest side. I've pretty much gotten the useful General skills to at least lvl 55-56, and important Mage primary skills are fairly close. But also remember that *gear* plays an important part of "powering" any class, and that's where my toon is very deficient. For the most part, I've been using mostly unstatted armor and accessories and an old T7 Henolid's Staff of Admiration (lvl 50) that I had stored away "for later". I do have a T8 Root of Nightmares I can use when/if I take Mage to lvl 60.
Third, and using Lightning as an example, an M/P is *still* vulnerable to any ranged physical DPS toon after rooting it, and *all* Rogue primaries who've bothered to level up Throw and Combo Throw. (Gee, those two skills aren't OP? Ask Rusty...) If the Mage is lucky and doesn't get killed by ranged DPS, then yeah, ranging in to stun with Discharge will help. Somewhat. You then have to back out of range and hope your other instants (IWB, FB) will take out the target before Discharge wears off. Presuming that Discharge hit or wasn't resisted or immuned against (Serenstum), that is.
I see it as a low-confidence prospect, at best.
As a P/M, my toon is also quite TP-starved. After all, I still had to dump TP into Priest skills *other than* Ice Blades as I was leveling the toon. There are a lot of those and they suck up a huge amount of TP. I leveled IWB as best I could using the remaining 100K TP orbs I had. It's not been enough.
And no, I am *not* going to use a Reset Stone and steal TP that I've put into IB. Being an eternal optimist, difficult as it is right now, I *have* to believe/hope that GF/RW will see the error of their ways and either reverse the 3-second cooldown to IB, or greatly mitigate it in some way.
But as things stand for me right now, M/P doesn't cut it in Siege and I'm actually more useful overall in playing the toon as P/S. I can collect merits from the castle balloons, sometimes from the middle zone balloons, and do trades with my guildies. I'm usually Herald, which allows my more capable companions to concentrate on what they need to do. When I get enough merits, I go for the merit/crystal discounts from the Library and then boost the offense/defense buffs from the Forge and Lumber Yard. I buy and set up defensive towers. And, when necessary, I heal gates that have been Tornadoed and/or scrolled, as best I can. (P/S is *not* the healing machine that some of the more ignorant people here seem to think it is, not when using mostly Mage gear and Mage stats. And I do *not* have the resources to acquire and stat alternate Priest gear.)
I don't *like* my current role in Siege. But it will have to do for the time being.