I agree, I fail to see why it matters, if Oak catches it, because again what about all those other character captures we'll never see again, or likely never see again. Who cares?
For all we know Oak if he does catch it, he might give it to Ash, for some odd reason, who knows, doesn't matter.
Oak has an interest in Rotom, so he wants to catch it.
If it makes anyone feel better, just imagine Oak being the umpteenth time we see a COTD capture a Pokemon, that we never see again.
Or it could be some Pokemon we've seen before return.