Bog standard Union attempting to raid a planet by traveling in normal shipping lanes?
Seems like it could stop one of those.
Interdiction on that means you need to have the capacity to search, which requires identification of the sort that says "Yeah, you know, that union looks more suspicious than the hundred or so others that go through every day."
(being as the Union IS the most common dropship in the Inner Sphere and periphery and many of them ARE hauling cargo instead of 'mechs).
If you're stopping EVERY ship that comes through, you STILL need more than a dropship with only six fighters and two smallcraft, unless you don't see a jumpship more than once or twice in a week (and then, it's STILL a good idea to have more than one of these, or something larger, just to handle the traffic overflow and the need for downtime.)
Customs interdiction needs either a ground base, or endurance, and if you're already building for endurance, you might as well build for coverage, which in turn means you don't go for minimums. (a single fighter squadron IS the minimum here-less than that and you might as well leave a sign saying "Please don't steal my stuff, I can't defend it")
On the whole, a force the right size for a raid isn't going to be coming down the known shipping route unless their target is your shipping, or they've got a pretty good bet you can't stop-and-frisk (Health and welfare) every dropship moving through. (this is how the Blakists were able to make use of their 'pocket warships'-they filtered into normal traffic and used obscurity to get into range.)
IOW it's a tactic of opportunity that relies on what? right,
insufficient coverage.
A dedicated raider in a raid prone area is going to use pirate points and mobility to get close and try to duck under or around your patrol. (thus, why you want both mobility, and coverage, and dropships, aren't good for mobility-it takes them DAYS to go from jump point to jump point under their own power-by the time even a quick dropship can react without a jumpship, the raid's already happened.)