Yeah this. ^ If it's a workmate and there are 10 other workmates coming, and they are ALL invited without their partner, then that's fine. If it's someone who doesn't know anyone else, or doesn't know people very well, then it's a really shit thing to do - only inviting them alone, and not a plus-one (IMO.)
Inviting someone, and not their spouse, when said person only knows the bride and no-one else, is terrible. This person must have been so lost and felt so awkward. I can honestly say that if someone invited only me (and not my husband of around 30 years,) I definitely wouldn't go.
If it's a married couple - or engaged - then I think it's weird, and bad form to not invite the partner. JMO of course. If you can't afford to invite someone AND their life partner, then don't invite the one and not the other.
It's all subective, and there is no right or wrong, but I think inviting someone and not inviting their fiance or spouse is odd. I'm entitled to think that, and I'm not wrong. Just as people are entitled to think it's not odd, and they are not wrong. An opinion cannot be wrong. And you don't have to be 'joined at the hip' to prefer to go to big family events like weddings with your bloody partner! Daft comment that is! 
Long running thread going at the moment about something similar here...
https://www.mumsnet.com/talk/am_i_being_unreasonable/5485488-wedding-invite-to-husband-only?page=1