Because you want it to actually go boom instead of ... not?
The 10/$3 18650s I bought from China were quite reliable in providing full power...for about a tenth as long as the brand-name ones. If it's just running a clock for a few hours and then a single pulse of near-dead-short to fire an igniter, I don't really see a problem. (And for that matter, it's probably easier to just buy a cheap clock with its own 1AAA power, then use its alarm line into a mini relay to trigger the larger batter[y|ies] powering the [igniter|detonator|whatever].)