It is, but using it would require us to use 5x as many ELBs as we need now (100 domains per cert vs 20). For some that isn't a problem and free is a great price.

20 domains on an ELB for a year, $10.80 per domain (ignoring traffic). Or put 5x as many on and get SSL for $2.16 per domain per year (also ignoring traffic).

That said, I'm really happy to see AWS head this way. Even if someone uses them, in min mind Let's Encrypt did its job - drive the cost of SSL down to where everyone can have it.