That raises an interesting question - I've been thinking about what role something like siege equipment really has in a world with with a bunch of mages running around. Is it worth building and transporting a bunch of mangonel when there's a bunch of people who can achieve the same effect just by waving their arms in the air? It's not very relevant for me, since I'm the only character disconnected from the world of incantation, but how widely disseminated is magic? When push comes to shove, what percentage of the general population can lob balls of fire or something equivalent?
Mages are used to supplement seige equipment, not vice versa. Sure, maybe a geomancer could lob a decent sized boulder capable of killing a man, but in no way could any geomancer throw a boulder far enough nor hard enough to cause any real damage to a castle wall, at least not while being outside of arrow range. Same with the other elements, the amount of concentration and strength required to throw any kind of elemental projectile comparable to a lob from a catapault just isn't feasible under seige conditions. Sure, if a group of mages worked together to throw a boulder they could do some pretty serious damage at a greater range, but most mages, even when working together, arn't capable of outranging seige equipment. Because for every mage you add to the equation the spell becomes more difficult to control, and it only takes one mage to slip up for a backfire to happen. It's hard enough learning how to control your own magic, but for every mage you add to the frey you have to keep their in check as well. You could have a group of highly trained mages accomplish this, and possibly outrange seige equipment, but it's just not worth the hassle especially when considering it takes a group of mages to accomplish the same thing a wooden catapult could do. Not to mention how tiresome it would be for the mages whilst the catapult can fire as long as it doesnt break or has ammo.
Theoretically a single earth mage could wreck some pretty serious havoc on a castle wall if he gets close enough, he could bore a hole straight through the wall, but there will also likely be mages defending the wall, and the moment that mage gets to work on the wall he would make himself a target for every defender on the wall, and it would only take a single mage to repair whatever damage he could of done. If mages were expendable it could very well be considered a valid tactic, and essentially just try to out-mage your opponents, but considering the rarity of magic weilders there just arn't enough to throw away each battle.
Essentially it all boils down to mages are too rare and valuable an asset to risk when wooden seige equipment could do the same job in a more cost effective manner. With all that said, covert operation utilizing mages is a fairly common strategy.
As far has what percentage are mages, probably around six out of every thousand. In this universe, at this time, an army of 3,000 people is a moderate size army (so not LOTR style with tens of thousands), then when you consider which of the mage populace are serving you as opposed to some other faction, an army of 3,000 would probably only have around seven or eight mages with them if they were fortunate, then when you consider you generally want to keep them alive from battle to battle, you tend to use them sparingly. Throughout the RP, when you see that a nation has a seemingly indefinite number of soldiers to throw away or there always seems to be a feral mage popping up, it's for the sake of keeping you guys on your toes.
When you consider the Tower of Mages is essentially a haven for mages, you'd consider their numbers to be quiet large, but in all actuality the Tower probably only has around 300 or so mages (fluxating depending on current events). In terms of mages, that's a MASSIVE number, but in the grand scheme of numbers it's quiet small. With all this talk of how valuable mages are, that's part of the reason Tower Mages swear to politcol neutrality, because literally every lord is gnawing at the chomps to get some more mages in thier service. Really, that's why Ferros, Valaria and Kain all stopped sending mages to the tower when they relocated to Valeal, because they would be sending extremely valuable assets to a country they despise, and they saw it as the Tower choosing to align with a kingdom (when in actuality they were just homeless and well, wanted a home).
Also when you consider that the tower regularly puts their higher ranking mages at risk to deal with feral mages, it's kind of stupefying to the outside world. With all this talk of how much mages are prized, it's important to not overlook how persecuted they are. Typically the common folk fear mages. Period. Nobles and Lords want mages for thier cause, but they also have the mentality: If you won't serve me, you won't live to potentially serve anyone else.
Tis a dangerous world for a mage.