Well, it's wrong in no small part because the financial industry does, of course, produce something of value. I don't really get how the hard core "finance is fucked up" people imagine that financial firms got started. Do they imagine they were just created by evil aliens or something?
Finance firms provide lots of very sophisticated ways for money to get from people who have lots of money that they don't currently have a use for, into the hands of people who have a need for money they don't currently have.
Note this is a weak claim. I'm not saying anything about the efficiency of this process or whether the financial industry is good or evil or anything. But you can't criticize the finance industry without at least understanding and engaging its outputs.
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.
Finance firms provide lots of very sophisticated ways for money to get from people who have lots of money that they don't currently have a use for, into the hands of people who have a need for money they don't currently have.
Note this is a weak claim. I'm not saying anything about the efficiency of this process or whether the financial industry is good or evil or anything. But you can't criticize the finance industry without at least understanding and engaging its outputs.