What's under it now?
Not sure, other then somehow you think it's to much work, that anyone would leave it in place.
By going over it you will have to deal with all the doors, door trim, baseboards, any built in cabinets being way off.
We have done it with no issues. I would however suggest taking up the old hardwood. If you go over it it would be hard to get rid of any squeaks in the existing floor and as the other person said, you would have to recut door jambs, and probably need to trim doors to make the height work. By taking up the existing floor you can then screw down the subfloor which will prevent a squeaky floor and make for a better finished product.