> string manipulation
That benchmark performs string manipulations that rarely occur in web apps. Web apps need: concatenation, substring, find/replace, maybe with regexps. All of those are implemented in C.
> memory allocation
Web apps don't tend to implement entire trees in pure Ruby. That benchmark is completely non-representative of real-world performance.
What exactly are you getting at? Of course it's easy to find a bunch of synthetic benchmarks that show weaknesses in particular cases. Still doesn't prove anything.