OK, if you were in control of the hashing algorithm used by Bitcoin, which one would you use and why?
An algo designed by Bitcoin Engineering Task Force
specifically for bitcoin. Then and only then bitcoin has a chance to be safe. Bitcoin should not use hashing algo because it is recommended by NIST or NSA or whatever. On the contrary, other organizations should use whatever bitcoin network uses because if it is broken bitcoin will act as honey-pot and will inevitably expose the weakness!
Roll you own = recipe for total disaster, see DVD copy protection and many other examples.
Well, how is that different from saying "Roll your own currency = recipe for total disaster"?
Putting together several standard crypto sub-systems to make something new, interesting, useful, etc.
versus
Creating your own crypto sub-system.
Using standard crypto that has been vetted by a worldwide audience of crytpo experts and has millions of hours of use/debug/hardening.
versus
Using crypto that is vetted by a very small subset of all crypto experts and has very few hours of use/debug/hardening.