Well, bashing of anything, religions or otherwise, is useful in that it may accomplish:
- Polarization different groups and a reduction of inter-group cooperation or harmony. Some folks actually intend to sow discord and chaos in their wakes.
- Shoring up and affirming their own positions or stances. As suggested above, this effect will also tend to happen in the maligned group (aka, polarization).
- Release bottled up emotions and frustrations that every human experiences. Whether or not this particular expression of release is healthy is debatable.
- Development of stereotyped propaganda that can successfully undermine the targeted group as many humans don't bother to do research and fact check.