When it comes to short quick bursts of data , the interface and cache sizes come into play, both the system caches and the disks own caches.
When it comes to data that doesnt fit in any of that stuff (aka 99% of everything :-) the platter speeds are the big bottlenecks , where the speed of the continual transfer of data is important.
I would have to say that everything advantages, the faster you get it on the platter, the sooner it is in reality finished, anything else is it still moving about in some aspects of the hardware. I/O is I/O, and can effect I/O in the rest of the computer. This becomes more important, when there is more than ONE thing going on.
The connection speed, is just as important to get it in and out and around quick and be done with that.
To the Platter is the matter:
If we are to specify things that are more effected by the disk platter speeds, and up to negativly effected by the stupid caching stuff, it is continual high data rate, Videos, large pictures, large databases accessed all at once , and when there is many other things going on filling and flushing these caches, simeltanious R/W I/O. If your really putting the computer to task, and doing lots of stuff at the same time, platter speed is everything.
Passing around in the rams and over fast connections
Things that fall into the cache size, and depending on the settings in the system, are most effected by the interface to the "ram" in the disks cache. so small quick I/O that is around 4-16megs. Regular sized picture, only the tiny videos, small docs, simple html, text, small files like parts of programs, dlls, plugs , and when there isnt 50 other things filling and clearing same said caches.
In these minimal uses of the computer, the caches are the speed, the rate you can get from/to those caches is everything. How long it takes to get to the platters, when there is not a lot of data, and nothing else going on, isnt really an issue.