PH-UKU
You appear to be an ATCO for a Class D airport. Let's say you allow a transit to some VFR traffic, and you have a dozen 737s inbound. Aren't you interested in positively knowing where this VFR traffic is?
There is also possible confusion with other traffic in the area, which happens to be lost. Let's say this is Gatwick. You could have non-XP traffic heading straight for your runways which, for the initial bit, is below Class D (below 1500ft QNH) and so you must assume it is OCAS. You might get worried about it, but you must still assume it is OCAS. A 100kt it will take that traffic about 1 minute to be overhead your runway.
If you do clear somebody for an overhead transit it helps to know their Mode C altitude together with their squawk.
I don't know how ATC handles this but I am sure they would rather know who is who.