There are a number of very good reasons to do this, but you are right, obviously two services as a rule cost more. I say, "as a rule", because the first reason one might wish to make use of two sims is the overseas trip. A lot of times it is cheaper to buy a local sim than pay exorbitant roaming with your home carrier. And yes, if one wishes, one can find a plan with web access and everything else the home carrier has.
Another good reason to do this is for backup. If one winds up at a deserted train station at the wrong end of town or with a flat tire somewhere in the boondox where there's no service, that could make for a rather bad day. Having a backup doubles your chances of finding service and also increases the chance of not being forced to pay roaming.
A third reason to do this is the peace of mind one has from not being at the mercy of his or her carrier. If the carrier decides to play god with their plan, the victim of that doesn't have to play ball.
And, then there's having a lump of humanity you talk to that eat up your minutes, but you were smart and for substantially less than adding minutes to your plan or paying overage, you got a secondary carrier with free unlimited to the carrier your lump of people just happen to use.
And there's the big reason almost everyone in this thread (despite any other reason) did this and that's simply because we're all hopeless geeks. And proud of it, my friend. Some people like to scuba dive and find lost coins; we like to push our phones to the max and find hidden functionality treasures.
Anyway, the reasons are endless, but most of them do not include ditching our carriers, although it does give us a bit more freedom if our carriers suddenly do something irreconcilably outrageous.
