Why use a localized ground plane?
It's not just all about loop area.  Small loop area is important to reduce radiation and susceptibility, but I expect you want your circuit to do more than not radiate anything.

Currents across a ground plane cause offset voltages.  That's bad since another job of a ground plane is to provide a common reference voltage to all parts of the circuit.  In a purely digital circuit, you might be able to tolerate a few 100 mV offset.  If the circuit contains analog components, much smaller offset could be bad.

Another problem with offsets are that they can excite the ground plane to resonate.  A microcontroller in the middle of a board with part of its power/ground loop running across the ground plane can turn the whole thing into a center-fed patch antenna.  The current loop may be small, but it causes voltage offsets at either end, which can cause even higher voltages at the edges due to resonance.

The more you use a ground plane, the less good of a ground plane it becomes.  You have to make a tradeoff somewhere that results in the best overall characteristics taking all the competing demands into account.