Of course many guests find snapping a few pictures a pleasant part of attending weddings.

This is not not a big deal for us and we are happy if they do so when formals are over.

When we are working through a pre-determined series of formal pictures with a timeline set, snap-shooters will slow the process.
A problem arises also when there are several cameras aimed at a formal group and the subjects will be looking at different lenses. The diverted attention will ruin your picture.

Another reason is when we are using a light-sensing trigger device on a second flash, other flashes will trigger this flash, ruining the pictures [that you are paying for] and again wasting time and energy.

Ideally, guests should show respect for the bridal party and their wants for professional photographs and put down their cameras during the formal session.