Questions about indexes with text_pattern_ops

Hi 

The database is initialized with utf8, so in order for LIKE to use the index 
on a text field, I used text_pattern_ops when I created it. So far so good. 

It's in the documentation, but there's no explanation of why this index will 
only work for LIKE searches. How come that I have to have two different 
indexes if I want to give Postgres the ability to choose index scan over seq 
scan on LIKE and non-LIKE searches? 

Is it a performance issue? 

Also, when I tried to create the index as a partial one (avoiding the 95% 
entries with empty strings), Postgresql chooses to use seq scan. This sounds 
counter intuitive to me. 

CREATE INDEX new_index ON a (b text_pattern_ops) WHERE b <> '';
This is 8.2.6.