Too quick and it makes those skills useless, though.
Ideally, there should be "cheap" skills that you can spam without improved mana regen (Charged Bolt and Magic Missile, probably), and then there should be expensive skills that you can only use regularly if you have the regen skills or crazy items (Blizzard, Tornado).
Mostly, mana was really bad in Diablo II. At lower levels, you never had enough of it, and at high levels, even if you never touched Energy and didn't bother with any Mana items, most characters would have all the Mana they could ever need. This meant that you had to slog through early levels with not enough mana, or gimp yourself with way too much mana later.