Actually the Druid is one of the most balanced classes in the Core Rules. Due to its weapon & armour restrictions along with alignment restictions it really drives the character down a specific yet flexible path.
With spella like Animal Growth, & Greater Natural Weapon a druid can make his animal companions realy strong, but at the cost of spells. Also as noted before you can only have double your Hit Dice if you stay in one area.
So it does keep the animal population under control, in a party siuation. And if your worried about a Druid's dire bear outshining a player in combat just remmber, it has a really low AC, lower in most cases then a player and will fall much faster and harder.
Animal companions are an avenue through which druids could maximize their combat capabilities, but from an RP siuation is every single animal you freshly reqruit gonna indanger itself? Animals are also built with a fight or flight mechanism. Don't get me wrong there is nothing wrong with having your animals fight for you but they should also on some level have a desire to protect you, and for you to protect them. Not just see them as a tool of combat.
Some other examples of ways people can get a strogn fighter to assist them, is Leadership.