There has been a lot of talk in Guadalajara about using smart cards on buses of the public transportation system, the only step taken toward this has been that a norm has been approved by the local congress a few years back.
So far, only the light rail system and the Macrobus use smart cards. The problem being that they are incompatible, which is at least problematic for riders that use both systems on regular basis.
The ideal would be that the smart card system for all three systems would be integrated, allowing riders use one card for the three systems. That would facilitate things for everyone involved, which is one of the main reasons to adopt the smart card to begin with.
Personally, I'd go one step beyond and allow users to pay with their smartphones. That would facilitate things a lot more, since a large portion of riders already use smartphones to begin with.
I rather use a smart card, specially if it allows me to use it on the three systems. I already use it for the light rail system, and the buses that it uses, and I really find it to be a lot more practical that having to worry about having the spare change to pay for the ride.
The public transportation system is a dire need of modernization, and the payment system is one for those things that would benefit the most. Both the light rail system and the Macrobus have proven that the smart card works, so there is no reason for the buses not to adopt it.