It's all about the "If".
Decoupling at the point of consumption may
be too remote, inductive/resistive-degenerated,
to stabilize the LDO. Many LDOs are more stable
with low quality caps than near-zero-ESR but
that falls apart at some point. It's even less likely
that one "client"'s decoupling can provide the
step-load current to meet load-step regulation
at another remote location.
Designers aren't going to model a complex PDN
in the course of design; if you're lucky the output
cap impact on stability is characterized and
component C, ESL, ESR bounds provided for board
design.