Computer, per se, are not bad. Computers infected with malware are bad. Infected computers can still do useful things for you - and in the background they'll be screwing you over also.
The argument I would like to make is not that finance is bad intrinsically - as you've said above. The real problem is that the current instantiation of this concept has grown a series of goals of its own, and to fulfill those goals it is doing things that use the common, shared, limited pool of resources of the global economy, siphoning off stuff that the rest of us could put to better use.
In other words, they could perform useful services for us, and they do. It's all the extra layer on top of that that's the problem.
In a strict, mathematical sense, the OP probably exaggerated when saying they are entirely parasitical.
The argument I would like to make is not that finance is bad intrinsically - as you've said above. The real problem is that the current instantiation of this concept has grown a series of goals of its own, and to fulfill those goals it is doing things that use the common, shared, limited pool of resources of the global economy, siphoning off stuff that the rest of us could put to better use.
In other words, they could perform useful services for us, and they do. It's all the extra layer on top of that that's the problem.
In a strict, mathematical sense, the OP probably exaggerated when saying they are entirely parasitical.