It means -1 turn cooldown. So a 6T cooldown ability becomes a 5T cooldown ability.
Apparently they'll be used for Soul awakenings on DMM soon-ish, but that also means we're talking about content 1+ year ahead for us.
So right now, yes, no use for them.
That would be Shingen.
Most veterans only use Defender weapons if they're SSR or dual-skills with Assault. Defender SRs are usually used as fodder, while Assaults are used if MLB with good stats. Some use 2 defenders, some use 1, some use none.
The best method of increasing your skill level fast involves understanding how skill levels work, so you don't waste fodder. However, skill levels are something that only make a difference when you stack a lot of them -- if you've just gotten your first SR assault from Disaster, you should probably focus on getting raw power first and worry about skilling up later.