[socalgal2]

PVS does not require a hierarchical representation. You can use any representation you want. In fact the one in the article itself is not hiearchical.

[Panzerschrek]

In practice many useful representation can be built only in a hierarchical way. Unless you want to force artist/map makers to split their maps in regions manually.

[socalgal2]

You can do it all kinds of ways. Artists don't do it, tools do.

Example: Racing game, since cars can't go off the track, you divide the track into segments (a few meters long) and compute the PVS for each segment.

Plenty of others.