Of course they should take pictures, actually it is usually inevitable. There is no problem with this, but possibly they should do it on a way that they will not disturb the professional photographer who was hired to do this work.
The invited guests mainly come to a wedding to celebrate, right? Those who are continuously busy with their cameras can not really be a part of the events. Not mentioning that with a simple compact machine or a cellphone, especially with low light conditions, it is impossible to take good quality pictures. It is also very annoying when for example the moment of pulling of the ring is covered by a hand with a cellphone or the photographer will not find room to take his shot about the first kiss because the guests block the way...