Assisting supplements social skills, with no explicit realization that the victim has been charmed, no resources expended, and no V and S components that might be seen. You are almost always better off just making skill checks than casting Charm Person unless you are really banking on the 'cannot be attacked' part of the spell.It doesn't obviate skill usage, it supplements the social skills. Don't have time to befriend the guard by conventional means? Charm him. Rolled a natural 1 trying to befriend the guard? Charm him. It's going to be more expedient in most cases than a skill check, and moreover can act as a safety net to the skill check. Also, if you suspect you need a high roll to succeed and/or might make the target violently hostile by your request, charming in advance will help with those issues.
And when you meet that guard again in a week, because many games are not a parade of one time characters that are never seen again, you're going to have a bad time.I never suggested that someone would do major favors for someone who has charmed them. My go-to example was of accepting a hot beverage from them, which just happens to be heavily dosed with tranquilizers. As I said then, when that person wakes up the fact that they know they were charmed will be probably the least of your worries.