well i think tunnelling is a crucial weapon if used well, and I dont see the MI having that many ways of dealing with tunnels, unless they are absurdly lucky, the only thing guaranteed to pop some units out of the ground is a nuke,
and, it keeps the enemy guess you can 'hide' units for some time and make the enemy second guess his moves,
However I do know one way the MI can really avoid problems from tunnelers, and thats mobility, lots and lots of it, wasp packs and anything else thats jumping more than 12 is a pain in the ass to catch,
more and more I prefer simply having my forces above ground so I can have more bugs, however I cant concieve of using a tanker above ground until your ready to attack, its a very good investment in that case and saves you from god knows how many hole punches, the same is true of the brain bug....