This would only encourage avoiding the fight. Hell we've got maps now like on baltic (or whatever it's aclled) where you'll get 2-hour off-map NOE raids by a lanc and a goon and a single fighter to deack, kill VH, and capture the V-base nearest the enemy HQ while the front lines are 6 sectors away, and with plenty of fighting at the front lines.
It has happened numerous times before.
A rule like this would encourage people to avoid the fight and sneak/steal the base without any opposition, while the entire country is 6-8 sectors away actually defending the fields under attack.
Bad idea.