If you took all the Bitcoin mining machines and made them work on science and engineering tasks they would produce hardly any meaningful output compared to the Top500 systems. The main reason is that Bitcoin mining is embarrassingly parallell while most technical computing algorithms require large amounts of communication. The difference between a Top500 system and a standard cluster is the interconnect.
Bitcoin's computational power numbers aren't relevant for another reason though: it's entirely confined to computing sha256, and only in a special way...
There has also been some discussion lately that Rmax, the performance indicator of Top500 has started to diverge from sustained performance. Sustained performance is a general term for actual science produced with a machine as opposed to performing well on the LINPACK benchmark.
Edit: There is also a huge difference running Gigabit Ethernet over short spans as opposed to over the Internet. Latency as opposed to bandwidth is the limiting factor for many algorithms. Light takes a lot less time to travel across the aisle as opposed to across the continent.
Tasks in bioinformatics "decoding human genome" is typically of this type, and are generally performed on clusters, even if a supercomputer is available. Where I used to study, applications for CPU time at the supercomputer for workloads that could run on a cluster was generally rejected, and directed to the clusters instead.
Machines take small work packets and crunch numbers, then pass back results in minutes. For many clusters, various steps of the algorithms require fast messaging for things like MPI.
TL;DR Bitcoin: no coordination during work