I would expect that, because companions are viable combat options, they don't get forgotten. But I expect they are treated like meat shields.
The Familiar shouldn't just be something tossed onto the first level of a class (or fourth level) and say "Here, this will provide some balance/frill." That's ALL the sorcerer gets. Meanwhile the paladin, unless he's a halfling or gnome, is unlikely to be able to take his horse into a dungeon. There goes that class ability.
Personally, I would like to see them as a viable option. Give up something to take them, and thus make them very useful when they are taken. As it stands, the wizard's familiar is a liability. It gives him a +2 to something, he can buff an ally in combat, and at higher levels it can scout. But it can do little else due to it threatening half a square and when it dies he loses XP. The Familiar is basically a mobile magic item that if taken away, is a loss to resources.
Make them a talent tree or what-have-you, and make them intrinsic to the character's abilities. The wizard and familiar become more alike as they level, meaning the wizard gets more traits of the familiar and vice versa. That familiar should be be a constant presence, and have a Benefit of being a constant presence at the table, rather than living in a pocket until it's needed.
Further, I think a companion can very well facilitate some concepts when it's offered as an option. Such as a Necromancer provided a Skeletal companion at level 1 as an option, so he has a personal little "knight" that declares him a necromancer, as they always have sway over undead, not just when they get Animate Dead; a summoner has an outsider companion that he can trade up as he levels, as summoners are always in concert with outsiders and thus he can make better deals.
As for the Paladin, I'm not sure what to do with him. The Paladin's general flavor has been so closely tied with "Knight in shining armor on steed". However, a paladin is often the "Two-handed swordsman smiting demons" or as James Wyatt said, "The shield-wielding guy holding back the monster defensively", thus it's not mount-focused. If the Paladin wants his mount, then he should get it, and from it springs mounted-related powers, because that's his character's theme. And that mount should be viable to some extent everywhere he goes. It shouldn't have to stand outside the dungeon, where all the action is, where it can easily fall prey to some dragon looking for a meal.
