Having several linking groups would be very handy with, say, 20 text layers with
a drop shadow and a glow layer for each. The current single linking group is
quite insufficient for such cases.

Each layer would be in its own link group, and clicking on the link button of
other layers would add them to the current layer's link group. When you unlink a
layer, it would revert back to its own link group. 

This would allow having a link group for each text-shadow-glow-combo in the
above example.