Also, after re-reading things, mounts count as Standard NPCs, while PLs and ACs count as Specials.
There's an upgrade to make purchased mounts count as Special NPCs. While not cheap, the cost is still coin instead of reputation.
This may be true, but also consider that if you are merely buying a simple horse with Animal Companion, you're probably not making the most of the Feat in the first place. The total XP value of the AC can be increased, and the basic horse could be upgraded as necessary. If all you want is a basic mount, buying one with cash is definitely the better option. If you want something more grand (and/or customized), then the Feat would be appropriate. The same could also be said for the Personal Lieutenant. At least that's how I see it.
I have considered it and it is certainly true that you can get more XP than a typical war horse uses. GM permitting, you could even spend extra XP towards improving your existing mount, though you would need 8 XP from 2 additional feats to actually match a coin purchase. You could even go so far as to get something that isn't normally on the mount list.
However, no matter what you pick, any conceivable NPC you could build would be outclassed by a similar one found in the wild by 4 Threat Levels; be it guards, men-at-arms, wizards, griffons, war horses, elephants, blacksmiths, courtiers, imps or dragons.
I am in a game currently with several characters who have Followers and Personal Lieutenants. They were pretty good at first level (pretty good, but not really overpowered) and even second level. Now, at level 8, they have pretty much faded into irrelevance. Attack checks, skill checks, defense, initiative; these have all fallen quite a ways behind the curve. About the only thing that has kept up has been vitality. The only one that sees any regular use any more is the Wizard as it can cast unlimited Cure Light Wounds so it makes sure everyone is healed at all times. The other ones are kept hidden away when anything dangerous happens or are just left home altogether as the risk is too high they will get killed and end up costing reputation.