i held (for a very long time) the opinion that if you start an album with a sound, you should stick with it....fans bought that album because it was different and they loved that sound...they didn't buy it for them to then change.
But as I've been in many bands myself I understand....writing in the same style constantly isn't rewarding, and you end up either having arguments because you keep saying "we've done this" or you try something really different.
people should try creating art for a couple of years and see if they can stick to 1 type/genre for more than 3 years......it's hard without getting very bored.