I can think of three purposes for a Familiar...
Combatant, Skill Monkey, Buffer/Debuffer.
Of all the Familiars I've ever had the one I liked the most was the Fairie (or was it Pixie) from Baldur's Gate 2.
The Pixie was a Rogue ina Box and made playing my Sorceror to level 28 and up a joy.
If Familiars became available perhaps we would be wise to limit them to solo quests... as Soloing can be VERY difficult for certain Casters at lower levels.