Manamune is a great item of its own accord, the only problem is Tear itself blows. ;/ But Jayce in particularly hardly has mana problems if you remember that being in Hammer Stance for a short while will quickly refill your mana bar, especially after his nerf if anyone besides me has realized there's no reason to level E anything but last now...
"Being smart with your mana" is a stupid attitude to have and exactly why mana/regen is so underrated in this game. You be "smart with your mana," I'll throw three times as many spells at your ass because I can and probably come out on top. Other than building from Tear, the only bad thing about Manamune is that you'll never hear the end of it from 95% of your potential teammates if you build it on anything but Yorick / Blitz / Urgot. And I even got harassed once for that last one. -_-
That 500 Mana on Frozen Heart ain't just for show either, there's good reason people don't buy it on Shyvana or Garen even though they love all the other stats.
They would need to nerf it quite more to made the E not worthy to be leveled second. And I believe pro top players still do not even pick W before having finishing they damage core. Moreover this makes you push, thus weak to ganks, and the mana regain power is low anyway and being in melee range for jayce is crippling.
Manamune is bad because you just simply trade your early game power (buying dorans as said Jebus) to have more mana to spam. And spam to accomplish what? more damage? I doubt so, as even if you can spam less, Bloodthrister will give you far more damage, same as Trinity. You'll tell me "just buy it all" and I agree but Manamune is too expensive, it delays your core too much.
And yes in my opinion it's about playing "smart", a good and experienced player knows how to threaten his opponent by his own presence on the lane, he doesn't need to spam spells and blow his mana. You need to make your opponent feel this "I am not bursting you yet, but if you come for this last hit, you know that I will punish you". It's like having the nuclear bomb, you need to be a "force of deterrence", not a crazy spammer.
Top lane, more than any lane, is all about the "intimidation game", being able to make your opponent believe "damn I can't trade with this guy" without being forced to be focused on the trading all day is the key here from my experience because it makes you win the lasthit war, and gives you the initation during the trade. They need to fear you !
* Just played Irelia against Darius yesterday, seriously I had one gank that gave me an advantage early, and there is NO WAY Darius can catch up with you after that (unless you're getting camped like hell and not cautious).