EXT4 cannot be resized while mounted. This is the main reason we use XFS instead of EXT4 where I work, we currently mostly use RHEL 6. This matters a lot for e.g. database masters.
This is surprising to me. I have resized many ext4 partitions while mounted (as / even). You can't shrink them. Yes. But you can grow them. I don't think shrinking partitions is a common operation and IMHO that alone wouldn't warrant a switch of the default file system.
