When feature classes should not be put into a feature dataset?

In general, it is a bad idea using feature dataset to group feature classes for the following reasons:

1) While accessing just one feature class, the whole feature dataset is locked.

2) When one feature class in the feature dataset is used, all feature classes in the feature dataset will be cached, not just the layer being using.

3) Don't put any layer/ feature class into a feature dataset unless you need it for geodatabase functionality (e.g. geometric network, topology, etc.)