What you just described has been the biggest problem I have with LH at the moment, though I disagree with your conclusion re: potential. I wouldn't even say that they're the key factor in early game, unless you have a fire/death apprentice, and their time of usefulness is quickly outpaced by trained troops' tech levels. Heroes absolutely must get a huge injection of XP gain in order to stay relevant throughout the entire course of the game.
care to elaborate? i'm not sure i understand your point.
let me give some examples for hero builds i like to use, that don't need potential or a very high level to dramatically change the game
- tank defender: grab defense 1&2 and you can use him as a tank. works like a charm against all those low level melee troops; not so useful against ranged units, though. just give him a few spearmen for damage; in battle, move the defender forward, tank 3-4 units, move your spears to flanking positions and swarm the enemy one by one. no casualties. bonus points if you play kingdom an have life 1 on one of your champs (regen to full after battle - grind mites, bandits, wolves etc. non stop). at higher levels (8-10 or something) he's by far the best option for dealing with dragons, golems, titans and other nasty stuff. doesn't really need more than 8 levels to tank. delaying those 8 levels for potential is just dumb, since he's a real force especially in the early game. later you can use tained tank units for this role. they aren't as good as a defender, but good enough to get the job done.
- life mage: pick up life 2 (if you don't have that yet), compassion, then life 3 (at level 5) for shrink, life 4 at level 7. at level 4 and 6 you can start picking prodigy. once you reach "savant" (should be at level 9) you can kick out AoE heals with no cast time and shrink nasty stuff (like dragons). chance of success is good, since you have 30 spell mastery from prodigy. at that point, he doesn't really need more level ups. can grab some water magic (if available) to have even more debuffs to weaken strong mobs; or progress into the summoning tree, or grab some of the utility spells that branch off the prodigy line
- death mage: go for greater necromancy, then death 3, prodigy 1-3, savant. requires about 8-9 levels. gives you access to skeleton hordes and allows you to cripple your opponents reliably with instant cast wither and mass curse.
- commander: get accuracy 1, command, then accuracy 2-4, tactician 1-3 and battle cry. if you have good support spellbooks available, you can grab some of those spells before finishing the main commander tree (air is great for a commander - mostly buff spells, so his low spell mastery is irrelevant). prodigy might be useful for him, but only if you think you need both, battle cry and whatever high level spells he may have access to. if he doesn't have useful magic, potential is a waste on him - go stragiht for battle cry (which you should be able unlock at level 10 or 11)
- warrior: get lethal 1-3; at level 5, he's already at 15 attack plus whatever his weapons adds. then go for axeman or spearman for extra AoE killing power, or head for bladerush, or grab the extra HP and chain armor proficiency and the swordsman line to use him as an offtank (he won't main tank really tough stuff, but unlike the defender, his buffed up counterattacks can actually kill
)
none of those builds require a very high level to be very viable. in fact, they are a lot less useful if you delay the early level ups for "potential" picks. let's face it - once you get to mid tier weapons, you can win the game without using your heroes. by delaying their devlopment, you sacrifice all the wonderful heroic deeds they might have performed during the most crucial time of the game (early game, obviously) to be a bit better at a stage of the game where they aren't really required any more.