That's why it's so important to find a nice Linkshell in FFXIV.
They will help you out when you can't find enough players for your stuff, help you with strategies, etc.

That being said, I often join random groups to help people out and enjoy a bit of nostalgia
Baby Ifrit, AFs, Toto-Rak..sure, I won't drop my evening-LS-program-party for these, but when I see a shout in Ul'dah and don't have plans in the next 30 min, I join

Incentives might interest a few people into helping, but overall I doubt the FF community is the type to do that.
Like Rayna said - whatever you give people for helping strangers is not as valuable as "playing with my friends only", "being the first on my server to kill XXX", "get drop XY from the newest raid", etc.