I have no idea why anybody would put anything except viewports in pspace.
Okay fair question. I have a 3D structure that's 40'-50' wide, 200' long and 50' high. Columns spaced 30' wide and at 20'-25' down the length of the structure. No two column lines have the same configuration, depending on where you are there may be 4 to 7 different levels of steel.
Now I have to annotate, four or five plan views of this model at 1/8" scale, 3 or 4 enlarged plans at 1/4" scale, two longitudinal and nine transverse sections at 1/8", misc enlarged sections in both directions at 1/4", dozen details at 1/2"(looking 3 directions), and twenty details at 1" scale (looking 3 directions).
To make sure the the annotation layers can be frozen in the right viewports I need a set of annotation layers (we use 2 one for text, one for dims) for each of the following:
1/8" plan
1/4" plan
1/2" plan
1" plan
1/8" section looking north
1/8" section looking west
1/4" section looking north
1/4" section looking west
1/2" section looking north
1/2" section looking west
1" section looking north
1" section looking west
That's 12 sets of annotation layers to deal with, making sure that each is properly active at the right time (pretty easy done programatically). But imagine the display of several tons of steel overlaid with hundreds dimensions and text elements flying at a multitude of levels and directions in the model. Each requires it's own UCS to insure it's proper location in the model and these require maintenance as well. Becomes a PITB to work with in a hurry.
Now only one set of these layers is to be visible in each viewport, so an inordinate amout of time is expended on keeping the layer display under control over 10-12 layouts. The PITB becomes regal.
Throw in a revision or two that relocate a couple of col. lines or changes the elevation of a plan, and the PITB spreads to vital organs
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Better solution: Model in MODELSPACE, annotation on the paper (in PAPERSPACE) requires only ONE set of annotation layers, NO UCS requirements at all and NO layer visibility issues at all.