Final....
You might well regard being non SSR equipped grounds for a refusal to enter Class D. Many of those who have raised this issue (access to Class D) are of a very different opinion.
The Reason I said non SSR equipped was to make the point that TCAS does not "see" non equipped a/c, making the task of the hypothetical B737 crew MUCH more difficult.
BUT.... just supposing the CUB has a transponder with mode C, it wanders into path of the B737 and a TCAS RA is triggered. A go- around is flown, safely, but at some expense to the PAYING Airline. This type of event could well occur regularly if unfettered access to given to all VFR flights.... end result? Demands from the PAYING airlines to turn the CTR into class A with virtually NO ACCESS to GA.
I repeat. CTRs ain't big. ATC will grant access when it is safe to do so and having taken into account all airspace users needs. Please note : It may not be safe to grant you access and the safer option FOR YOU may well be to spend perhaps 5 minutes more routing around the CTR. Or is a 5 minute saving worth the risk? Not in my, or any ATCOs, book
Rgds BEX