User blog comment:Xilinoc/Ch. 642 - BABY, HOLD YOUR HAND 5 (EYES ARE OPEN)/@comment-1333438-20150911015800/@comment-1338810-20150911021000

Actually given what we learn in this issue, Mayuri probably risks Nemu's life so often because he's endlessly confident that his greatest creation is so badass that she's not in any actual danger. If Nemu survives whatever her drags her into or uses her for, then it maintains her as Mayuri's masterpiece; if she dies, then she wasn't that great after all and he moves on to make the eighth, even better Nemu.