If you want to beat the game in the fastest time, you should really try to utilize the various mana and rage filling options and tricks. It is imperative to end the battle with as much mana and rage as possible. However, the warrior has easier time maintaining that...
Still, do not think that this is a weak point in the design. There are tricks to fill up your mana, there are mana potions, there are various items, there are green dragons, there are mods, and there are cheat codes to fill your mana. I do agree cheating is bad, and if you're trying to beat the game as a mage, you have to live with that mana disadvantage (maybe that's why I don't like the mage that much). It is in the game, and if that is so, this is the proper way it should be played. Otherwise it would've been patched.
It's easy to point out design flaws, whether the claim is justified or not, it's another thing to try and deal with them. It's easy to stomp your foot and proclaim something's wrong when it doesn't suit you. The mage is undoubtedly the mass killer, that can easily reduce the enemy's army stacks equal or lower than his own in record time while losing few to no troops (geyser/fire rain, underground blades, time back), but he has to suffer some penalty for that - otherwise the game would just be way too easy for him when he gets up to the point where you have 150+ mana, 30+ intellect and a big boost to the damage of your spells.
In addition to that, beating the game in the fastest time requires a lot of effort and a lot of rounds dedicated to recovering troops (sometimes over 60 rounds worth making mana for resurrecting - with the paladin against Baal on impossible, the battle went up to round 143 before I can fix all my troops and get back to full mana + rage), rage (through the inquisitors) and mana (chargers, thorns + poison cloud + magic spring, etc). That's just the way it is done. If you're really striving for a quick victory, you should learn to live with that
EDIT: If you're complaining about that now, I wonder how will you feel when you get your hands on KBAP. Faster mana recovery (Meditation as we know it) is tier 4 skill with a lot of prerequisites and mana recovery in battle (Concentration as we know it) is a last tier skill, also with a lot of prerequisites. That usually means that you can't get lvl 1 in those before level 9-10, while in KBTL, you can get them in no time. And even if you do try to get them asap (say, lvl 7 - that's when you're on the second island with no easy access to archers, inquisitors and royal thorns), you deprive yourself of having Distortion magic in these levels (because they're in the order magic tier) - and by that time in impossible, you're already fighting "overpowering" and "impossible" enemies 80% of the time (where your fire arrow just doesn't cut it and you need chaos magic to get fireball, which often isn't enough as well), and of course, winning the battles with no losses is imperative for the Leadership awards. Suffice to say, a mage, striving for fast victory is having way easier time on impossible in KBTL than in KBAP. So once again, if you guys dislike it now, I wonder what kind of uproar there will be when you get KBAP.