How is it reckless to not support killing an entire group of people for something they didn't even do? If Meredith ordered the death of every Ferelden in Kirkwall since Anders came from Ferelden, would you consider it reckless to fight against her then?
Also, Gaider said that any mages who survived an annulment are made tranquil, so all those mages you spared were condemned to a fate worse than death, not to mention it is made clear that practically every mage in the gallows was killed if you sided with the templars.
If we were to look at the the Kirkwall Mage-Templar situation a bit more realistically. There is no way a Mage supporting Hawke would have walked out of that alive. If anything he would have simply died a maytr.
The mages are small in number compared to the Templars.
Templars are trained for combat against Mages.
Templars can nullify magic.
The Gallows was cut off from the Mainland.
Hawke had a small band of friends and a small group of mages, some of whom did not wish to fight the Templars. Which means that Hawke and his merry band would have been responsible for fighting every Templar to save the Mages, whom would have ended up dead anyway, making the fight with the Templars entirely pointless.
Realistically speaking, there is no way the Mages could have escaped the Gallows, it's entire design is built on offering very little if no escape options. The ending slides about saving a lot of Mages if siding with Mages is ridiculous. How did they even get out of the Gallows? It's surrounded by water and you need a boat and I doubt the Templars would leave boats just sitting there for Mages to use to escape. if they did, then how many Mages managed to fit into the boats? Sounds like a big plot hole.
And no matter how good Hawke is, he is severely out numbered and reinforcements (Zevran et al), don't even show up until the Meredieth boss fight. If Hawke is a Mage, he's screwed even more because there is no way, Him, Anders, and Merrill would have been able to combat the Templars using anything other than Blood Magic.
Overall, it would have made more sense for a Mage support option to be for Hawke to deny the Templars aid, then try to escape with his life rather than actively try to rescue the doomed Mages.
Who cares if they're made Tranquil...if they were. At least they lived.