I would never comment on a leash being weak...
It's helpful. If you're on someone who can't do much of a clear without a stronger leash it's definitely expected. That doesn't mean you need it though. People have reasons for not leashing. Not that they're good ones (I'd prefer to leash than give myself a small edge if i have to choose).
E.g. a whole ago i had a j4 jungle, i was hecarim top, red side. He started gromp so i could start wolves. Bot got in to a fight near dragon before minions spawned, they ended up not leashing at all. J4 said nothing, and after i did wolves and he did gromp i came and used a second flask charge and gave him a few q's on blue and tanked a hit or 2. Took me like 6 seconds, helped him a lot. He said thanks - but didn't comment anything about bot not leashing.