Like most others here, i almost exclusively use it as a crop tool, a very hard to use precisely croptool, but that could be fixed by the suggestion by Rob B about crop to layer/mask
And as J Bills mentions i have also never found any use for it to speed up a rampreview, but to fix that i guess needs a heavy reachitechturing of the way the AE render-engine is working.
I used it more in the old days to save ram when i worked in HD+ on a machine with 16GB or below, but that is luckily not very often happening anymore, but might get a use again with the 4K project that are slowly beginning to come in. :-)