Not any code can reuse any other code, but patch clauses mean code can't even be reused in code with the *same license*, prohibiting it entirely. I hope you're wrong and that "code reuse is unimportant and can be prohibited" wasn't really the rationale.
I think (but I am not sure) that this patch close was to use TeX (the GNU project use TeX in an essential way although RMS had said he thinked for long before accepting it; at the beginning Debian had strong relationship with GNU).
This patch close is almost equivalent to say that code reuse is prohibited. If this close is explicitely authorized in the DFSG, we cannot argue that code reuse is an essential DFSG-freedom unless this clause is changed.
Olive