I do see the benefits of not having signatures enabled by default, and Caddyman brought in several that I never would have thought of, but tome it comes down to what the purpose of the feature is and the purpose of the forum.
This forum, like most others, is a place for varied individuals to come and discuss many things and just be themselves (within certain confines).
Signatures are a vital method to highlight something about yourself that doesn't require people to enter your profile page to see, that isn't by default listed on your member info. Some use this for quptes, some for map records, some link to their favorite posts or call attention to upcoming events.
My signature is basically non-existent, and I'd be perfectly fine with not having my signature, but I still think it is wrong and against the whole purpose of our community and the purpose of signatures to default them at off.
I don't know how many users browse mobile ( I know I usually do and I've never had a problem with the site) and I don't know how many users will be fine with default off but switch theirs on, but if that number of users who switch signatures on is higher than those who knowingly leave it off (again, opting into signatures will result in many people simply not taking the time to make the decision at all and going with the default) then there is no good point to defaulting at off.
I ask that people consider whether they personally would leave signatures off, or if they'd switch them back on anyways, even if defaulting at off doesn't bother them.
Because I'm sure we have a good number of polite members here who would be okay with defaulting at off but would themselves immediately wswitch signatures on anyways. And that kind of sends a mixed message doesn't it?
On paper it will look like people don't want signatures on, but in actuality they would.
These are my opinion, my hypotheticals, and I don't mean to assert them on anyone.
But I sincerely believe that with this and most other features and systems, defaulting at off is absolutely the wrong choice for the greater enjoyment of the forum.