technically, you're right. but i don't think using a non-TBBU memory module is an impossibility. i could test it out once i get around to repair some of my PERC 5/E cards (i still need to de-solder some capacitors to measure their specs so i can order the parts, so it's not anytime soon).
I think the memory is required even if running with write-through cache, so these DIMMs could be useful for that if your current RAM is bad. A PERC 5/e with a standard DIMM running write-through & drive cache off will be secure and may even still outperform onboard fakeRAID5 solutions. A TBBU would make using the PERC much more worthfile though.