We did a bit of that, it's more about balancing them out.
Stongheart the Paladin meets NPC's as he progresses and can summon 3 - Elkhorn the Dwarf, Peralay the Elf, Ringlerun the Wizard (he can summon them all at the same time, which means beginner players who want an easier game should use Strongheart). He meets Elkhorn right when the game starts but the others come later.
Our Mage, D'Raven, can learn a Summon Imp spell that only he can use.
All the players can hire Sarken Mercenaries, and later they can steal a dragon egg and a trainer hatches it for them (for a fee) and they can then summon a baby dragon helper.
Each character class is unique, they all can do special things the other ones can't (and all are true to the original D&D rules). For example:
Paladin – Cure Light Wounds, Turn Undead, Summon Allies (Elkhorn , Peralay, Ringlerun)
Dwarf – Resistance to magic, can find Axe of Dwarvish Lords, command Golem, infravision (see in dark)
Mage – Cast wizard spells, use magic wands, can learn Necromancy Spells, can’t use swords
Priest – Cast priest spells, Heal allies, Turn Undead, find Mace of St. Cuthbert, can’t use sharp weapons
Thief – Steal, Hide In Shadows, Backstab, double-jump, Unlock metal boxes & disarm traps
Elf – can use Triple Bow, cast some wizard spells, can learn Druid Spells, use swords