I knew of it, but didn't follow it, and assumed it was more due to Brazil's general ineptitude than a lack of support for the Paralympics. So the Olympics have to support the Paralympics - seems fair enough.
But it does improve grass roots participation. And just because it's not easy to count...
Well certainly not the latter. Off the top of my head, the main benefits I can think of are (in no particular order):
Giving disabled athletes the chance to compete in sport.
Giving hope and pride to the disabled spectators.
Encouraging disabled spectators to take part in sport, which is healthy...
The value (worth) is that...
Obviously it's not perfect, and never will be, due to the points already raised, but so what if a few disabled people profit from it, if at the same time many thousands of disabled people can take inspiration from the athletes.
Well obviously. There are rewards for those who succeed, of course people will try and gain unfair advantage. It's also particularly difficult for the athletes who find themselves on the wrong side of the cusp between categories, so they have no real chance of winning against those in the class...