If each function is in its own section, then all function calls would need to be indirected through the PLT/GOT, even function calls within the same translation unit? Ouch.
No - the linker is there to resolve references among sections and can do so without PLT/GOT indirection when creating things like static archives.
There may be a code size cost in some architectures - that since the call destination can be relocated far from the call site that the assembler will need to make sure it allocates enough space to reach the call target instead of a small PCREL relocation.
