I also agree. They can be used as a weapon and probably very effectively even, but they are primarily meant as a benefit for your own cities.
That said, the best situation would probably be to selectively put some corporations in your own cities, but also put some in the cities of your colonies and vassals. Not to destroy their economies, but to reap the HQ costs and help them at the same time. I think this could be very effective for both you and the vassal. After that you can even try and irritate some of your real enemies with your corporations, but that's icing on the cake.
The great thing about corporations is that it can be anything you want.
If you have some productive tiles and some money, you can put the corporations on some cities.
So with mining co. or creative constructions you can put them in cities that have more multiplies such as the iron works, heroic epic or cities with military academies.
If you have many trade agreements and your economy is very much a commerce based one, you may be able to afford to spread your corporation to 80%+ of your cities, because they are working more commerce tiles to pay for the corporation.
The great thing with the concept was that is is very multi-dimensional.
So for example if corporations cost 40

"well if I can get each city to get 20 extra commerce at least, and make BMG then I could pay it off and make some profit from the HQ".
"if I can get each city to get 15 extra commerce, then with the HQ as well I can break even".
The problem I have isn't that it costs a lot, but that the costs change whereas my income stays the same.
So this same thing that I calculated would be profitable now, in 30 turns time Ill be thinking "oh crap those corporations are just about breaking even, damn I shoulda gone SP/CS so I dont have to deal with all this crap and also deal with diplomacy for resources"
and in another 30 turns you'll be thinking:
"damn those corporations are costing me 120

Wouldn't it be great if the cost of corporations were fixed so that we can depend on them?
Even if they make it expensive I would understand (although then it would be more like Science Academies and Mil. Academies by being very selectively placed instead of potentially being both a selectively placed model (for semi-productive semi-commercial countries and a whole new economic system for fully commercial countries.)