Tips are usually forced and small, and don't scale in time and space. Value-for-value payments are voluntary and open-ended in amount and time, which makes them scale.
Discussion
I'd argue we don't even know what zaps are yet, and zaps also won't be the end of it.
Read dergigi.com/busking and dergigi.com/vew for my long form thoughts on this, as well as value4value.info - and listen to Adam Curry. He's the most prescient on the topic I'd say.
It’s like an inverse invoice. You deliver the value and don’t except instant payment. You let the value be priced by others. Pure freedom from contracts. Probably only works for open source like things
Zaps are a game changer, as much as people want to send value when they receive value, people also want to show others that they have sent value, sending tips and then having to overtly announce you’ve sent value looks like your seeking attention. Even though people are, they don’t like it to seem that way, the automatic recording and visible nature of zaps sets it apart.