For me, it's the economics of time and motivation. I've got too many shelf queens staring at me in my workroom. Almost all my projects involve super-detailing and often, heavy conversion and/or scratchbuilding. I've had to realize that unless I get in a long groove with one project (getting it ready for painting), that it may be de-railed by loss of motivation, a research gap, waiting for some pictures/documents, etc. Thus, I try not to spend extra time on sub-assemblies that can be completed by the expenditure of a few dollars -- such as the addition of accurate, pre-made periscope guards to my Allied tank projects. Plus, they're much more symmetrical than I can scratchbuild myself -- win-win scenario!
I need to use that energy on other trickier assembly projects!