Some thoughts:
1. Cohorts can significantly improve a character's/party's 'combat readiness' (providing muscle for a Wizard, Buffing and medical support for a fighter etc). This is particularly useful in smallish parties that might otherwise lack some key functionality.
2. 1st/2nd level Followers that fight alongside a 10th+ level party get killed. Wholesale. So that ought to be discouraged. Plus, having a gaggle of low level sorts mucking about the landscape just adds a massive amount of dice rolling and paperwork without accomplishing much.
3. So. Spread 'em around: A couple of loyal Followers can be trusted to manage paid hirlings (3sp/day!), and can staff the character's 'base camp' (castle, chapel, manor, keep, city household). Another Follower (again with paid staff) could operate an Inn, merchant vessel, or other 'useful' business. Followers can be the character's 'agent in place' inside a palace, or (brave!) powerful criminal organization. Another couple (suitably supported) could serve as squires/men at arms looking after the PC's mounts/camp (but leave 'em outside the dungeon!)
4. Followers are an amazing resource for the DM: They provide a ready avenue to 'feed' campaign info to the players, and can speed up a lot of otherwise 'boring' campaign activities (fencing mundane loot, interfacing with tradesmen, stocking up on supplies, gathering information, etc.)
5. MOST of all, they offer endless roleplaying opportunities by providing a regular cast of characters that can develop depth over time.
A'Mal