Constructive & Helpful!
Lightmage isn't much of a mage with the current regen model.
Mages in fantasy usually covers a wide variety of magical powers. Necromancers are mages that specialize in dark arts, Shamans are usually specialized in nature based magic (or in AQW's case, elements), Cryomancers and Pyromancers specializes in a certain element and then the other multitudes of different "mancers" you can come up with. The first 3 are still "mages" in design, so to speak, even if they don't have the Mage regen model.
Then you have classes that fall into half mage territory like Paladins and Spell Blade type classes, Dragon Knight being the most prominent example in AQW, these could probably be excused.
Lightmage/Lightcaster are both still mages that specialize in light based magic and their counterpart the Dark Casters who specialize in darkness based magic. Although I couldn't necessarily describe them with any usual means, Lightcasters and Lightmages would be kind of like paladins and Dark Casters would be sort of like Necromancers, Arcane/mystic Dark caster even summons a dark minion.
Enough silly talk and on to the serious topic, Lightmage is a fairly strong class and does "only" cost 1k ACs + eventual Lightcaster + an armor on top, if you just like the design of it. So you are getting 2 classes + an armor, even if one class becomes obsolete later on. Even with the Warrior Regen model, the class is still easily playable and can get you through a lot of things at a decent pace, not even considering the support features it has, which gives it even more value. While the Warrior Regen does make it a far inferior class, I think it was a good (if surprising) change, because the other alternative was to literally nerf the numbers into the ground so much that it would just be another average/below average class, which is not what you want an AC exclusive class to be. You wouldn't want it to be like Undead Goat where mana isn't an issue (usually), but the damage is so horrible that any decent class will outpace it. In 2018 AQW where DPS and/or big crits is the name of the game, that is the last thing you want the class to be.
I always advocate for having Warrior regen on classes and Rogue regen on heavy dodge classes of course, as long as it's done correctly. An example of this done great is Ultra Omniknight, where you have to think a couple moves ahead and consider your available mana, the class doesn't feel like it's stopping you from playing it due to bad mana regen. An example of this done poorly IMO, is Cryomancer. Cryomancer is one of the most unplayable classes in AQW, it feels VERY sluggish and you run out of mana in a litteral instant without any real way to get back up to a usable amount of mana quickly. Now, Ultra Omniknight has more or less equal mana costs to Cryomancer with the same regen model, so what is the difference? The difference is that Ultra Omniknight's cooldowns give you time to breathe, whereas Cryomancer bombards you with active skills just sitting there, waiting to be used when you clearly don't have the mana for them. Even playing conservatively, Cryomancer doesn't feel fun to play, because the Warrior regen model is not giving you adequate mana to work with. Ultra Omniknight's haste and higher crit rate is certainly also a factor.
I can name other examples of classes where warrior regen doesn't ruin them, like Necromancer (although that's due to having a special mana gaining ability), Dragonslayer General (vs non dragons), Shaman/Evo Shaman, Collector (but not VoT) and the class currently on everyone's mind, Lightmage.
I would still recommend Lightmage and eventually farming for Lightcaster, if one was to buy a class for ACs.