It's more the free-ness over the easy access. There is only one free certificate provider, StartSSL, who I have had my cert from for the last few years, but it always seems to be a right hassle to get it done with them.
Also, for the less savy users, it might be a struggle to find the right key and the certificate and paste them in to FreeNAS as some cert providers don't give you the certificate in an 'easy to use' format.
It's just something I'd love to see and don't see a reason why not (time permitting of course) considering it's going to be totally free!
I was able to generate a Let's Encrypt cert on an Ubuntu server, then transfer it across to my FreeNAS box, but that's a bit of a pain as it requires changing of the A record to the server you're generating on, then changing back when done! I did give it a quick try on FreeNAS itself, and I'm sure I could get it working, but it was missing a bunch of dependencies.
P.S - I wouldn't be that bothered if they provided 1 year certs as I'd just fire up Ubuntu every year, renew and be done with it. However, the idea behind Let's Encrypt is that you renew, using an automatic process (some sort of script I guess) every 90 days (I think it's every 30 or 60 while in beta). Don't know why, but that's the way they're going to do it apparently, so again something in FreeNAS that would auto-renew would be amazing. I guess this might get more traction when it's actually released & more people are using.