You are definitely on to something there doctordun. I wonder the same thing, and have tried out a few batteries from a couple of places. In fact, I'm about to checkout my first Halo very soon
I try to stock up on cheap sale cigalike 808s, and found (with buying a number of cheapos of one type for example): some can be dead on arrival and there can be plenty of variation in the performance of the batteries. I've noticed a few different 'generations', it seems, of one brand of battery, even just in a year.
And even with higher-priced batts n such there seems minor, but noticeable variations in design and functionality. I've had some last days, weeks, and many months, and one real winner batt is going strong still... It is nice have backups and flavors galore around though for sure.
All that said, I've had good luck with a few classic brands, for my buck I reckon - despite a few duds and refunds (customer service has been fine across the board). I haven't tried a Halo, yet..
In general, like everything - you get what you pay for, though there are bargains to stock up on - sometimes that overripe fruit is the best! I'm pretty excited to see what's up with these Halos.
Anyway, the way I see it is that yes, these things are probably fine to sit unused for some time, I don't see any problem with having a few extras around, especially if the price is right. Though, I try to pretty much alternate my batts often enough so they don't go for more than a day or two on average without a use, but I haven't noticed a huge difference in ones that have sat a few extra days, so I'm less convinced about that one.
And as DaveOno describes there will be some performance decline over the long time periods even with the best maintained batteries.
ps I just powered-up a 'new' lil 65mm I got in June, thing charged up and is working like a champ. I am pretty weary of pre-filled cartomizers too, speaking of storage...