There is a three day window for the Government to go to F.I.S.A for a warrant in cases that require immediate attention. I think that is more then enough window. The only reason to avoid a secret court warrant in such a case would be if the domestic wiretap was done absent probable cause and the wiretap itself failed to produce probable cause for the Government to have done it to begin with.
How can you ask a court for a warrant for a linetap that you have already done that produced no probable cause for doing it in the first place?
The fact that you, say, intercept a phone call coming into a person suspected of having terrorist connections, but not yet under warrant, from a foreign country (like Afghanistan) does not give you time to go get a special warrant. You would be two days and 23 hours, 59 minutes late. Sorry, I don't want to take that risk.