If the base target were [something like "pm_display"] instead of "_new", then users of fullpage chat would get the same behaviour as now
No, it wouldn't. If pm_planet was used for target, the first followed link will open a new window as if _new had been used, but the second followed would overwrite the window opened by the first link (if it's still open) instead of opening a new window.
For example, if you click on each of the following without closing newly opened windows, you'll end up with 3 windows instead of the desired 4.
<a href="?node_id=67275" target="_new"> (jZed's home node)
<a href="?node_id=381608" target="_new"> (my home node)
<a href="?node_id=67275" target="pm_display">
<a href="?node_id=381608" target="pm_display">
I *hate* windows overwritting each other. I've lost much this way.
Update: By the way, _new is non-standard. _blank is the standard target to specify that "the user agent should load the designated document in a new, unnamed window."