Pee-ews
Then, there is the issue of prostrations. There is something unwieldy about having to slide out of the pews in order to make a prostration. If the church is small and the service fairly well attended, the available space around the pews becomes an issue and a prostration, an exercise in body control not to bump the person next to you. At times, people get discouraged and not even try.
I still consider those valid reason - perhaps primarily so, but recently another reason has been coming up in my mind: rigidity.
Pews impose a rigidity on the congregation that seems artificial. People go to a place and remain there for the whole service. By virtue of being human and simply needing to move your feet even a little bit, a lack of pews will will create some random movement. You don't finish the service in quite the same place you began.
This hits home for me as a parent of small children. In the context of normal human motion, the motion of children within the service is not an anomaly. Yes, it may happen on a different scale than the motion of adults, but it is part of a continuum. However, if the standard is the artificially imposed rigidity of pews, then any motion begins to seem out of place and children can begin to seem out of place - perhaps not even consciously. To me, that stinks. Pee-ew.