Indexing of Data Warehouse tables?
As long as the indices are smaller than the original table, or sorted differently, they can offer a performance boost, though in practice if you have low cardinality they may not.

The main cost of indices is increased time required for inserts and updates.  If your database is updated daily but read from frequently, even large indices may be worthwhile.  It depends on your data.  Remember: all generalizations are false.

Can you give a more specific example of where you're considering applying an index?  If you've already created one, does it show up in execution plans?