A few thoughts about the cyclist with 18 rear blinkies:

1: If he ever gets hit from behind it probably wasn't an accident.
2: I use discount-store AAAs that cost ~ 1.97 CAD for 18 of them, and I get about 3 or 4 months of run time out of my blinkies with them. If I had 18 blinkies that take on average 3 AAA batteries each, it would cost me between 18 and 24 dollars per year.
3: I would bet you get some funky patterns as all those lights goes in and out of synch over time.