Do you HAVE too? No. Should you? Absolutely,I would just go ahead and use the wall paint

I have 2 quarts of Behr flat white latex "ceiling paint". It's a small room (with sloped ceiling, but hopefully won't take more than 2 quarts since it's primed, I'm going to use 1.25 - 1.5 gal of Kilz between the ceiling and walls - and I only have 2 quarts of the S-W wall paint too!). Thanks.

Just wondering, why "should" I? Except that Kilz doesn't have M-1 added that I was going to put in the paint.

The primer is just that, a primer, it was(is) not made to be a finish coat to withstand dirt, moisture, etc.

Talked Mom into 1 coat this morning. We'll see how it looks tomorrow but I have to do walls so 2nd coat on ceiling may have to wait til Jan. But now she's asked me to do the other bathroom's ceiling! She was just worried that sheetrock would pull away from ceiling (or texture would pull off rock) b/c original 1970's ceiling was textured, not sure if painted, but then more texture sprayed on (when all I wanted was contractor to do taped joints where he reframed for shower stall), then 2 coats of Kilz she thought 2 coats of paint would break the camel's back!