I preached on the parable of the faithful and wise servant (Mt 24:45-51) on the weekend, and found the commentaries pretty pedestrian (a common fault when it comes to Matthew, I've found...). Call me pious, but I'm of the opinion that Jesus doesn't waste his breath, and so if he felt the need to say more after the parable of the thief and the householder that precedes this one, then he must have had something to say other than 'be ready' again.
So I dug around for a while, looking for something that made this a parable and not just another example or metaphor, and I think the answer's in the 'household' bit. That is, Jesus isn't just posing a question about his own disciples, he's also taking aim at the Temple leadership, prompted by the assumptions his disciples appear to display at the start of ch. 24. They look like they're being faithful because the Temple's in great physical condition...but Jesus says a faithful servant looks after the other servants, not the house.
Plus, there's also the fact that ch. 23 is a fairly serious hosing for the Jewish authorities for their failings in precisely this, and Jesus concludes with the reflection that Jerusalem's house, the temple, has been left desolate by them.