Somehow it seems the most important thing has gone unmentioned:
- Lock up 50 mind-controlled muggles in one cave and lock up 50 inferi in another cave.
- Keep locked up for 20 years
- Return to inspect results.
I am pretty sure you will find your 50 inferi up and running, but your 50 muggles will be positively a stack of useless bones. And as magic can't create food1 your best option is to just use defenses that do not require upkeep.
But there are other considerations as well:
Draw: The fear factor
As mentioned by others already resurrecting someone you personally knew as an inferi is definitely not a nice experience. There are two important things here though:
- Inferi look so little like their former self that you are likely not to even recognize them
- And even if you recognize them you will just be more motivated to put them out of their misery rather than less.
And on top of that all uses we have seen of inferi were impersonal, so it seems the psychological attack vector was not really considered by the dark mage world. All that however does not mean that it doesn't creep anyone out when a half-dead monster appears, or that one wouldn't be extremely afraid of being turned into a half dead monster when fighting Voldemort.
Con: Fighting power
- Throw 50 mind controlled muggles that can not feel pain and 50 inferi in an arena
- Watch the free for all
- Observe last man standing
As long as we didn't tamper too much with the minds of the muggles (just so they don't feel pain and are determined till the end) I think it's a fair assessment that the muggles will easily win. Traditionally zombies have two strengths:
- A single scratch is enough
Both things do not apply in such a scenario, so the muggles would have to mess up badly to lose. If they wouldn't be mind controlled then they might have a bit of a harder time, but in the end you would need to mind control them either way if you wish to use them as your long-term loyal guards.
In conclusion I think it's fair to say that the reason they were used was not because of their strength against wizards, but the fact that they could stay there for a hundred years and still be ready to fight.
1 Well, there are ways around that (multiplication, enlargement, etc.), but lets just go with the standard story telling version.