That has happened in the past, when the P2SH proposal was batted around, pools voted, and people did switch based on their politics. More often though, miners

That has happened in the past, when the P2SH proposal was batted around, pools voted, and people did switch based on their politics. More often though, miners will have a main pool and backup pools based on uptime. In the scrypt mining world, pool hopping happened (happens?) for economic reasons; hash -> litecoin variant -> USD (or BTC)-denominated return is continually optimized. This caused some wild swings in mining difficulty for some scrypt-based coins as their exchange rates changed.