Regarding the theif only using light blades. If that irks you, then you have a right to be irked. Personally, I would take light blades anyway...its just such a good, well supported weapon group!
Regarding this Slayer > Theif debate. I gotta point out our groups rogue, which is a cunning sneak build. Mobility is key with that character and also to the theif, and to say the theif is less than the slayer because the slayer can average out more damage and has broader weapon groups is a very shallow argument.
Every class and build is a measure of the whole, and to focus on numeric comparisons of subcomponents wont get you anywhere. The way our sneak can weave through combat and hit the right target is phenominal. All the damage in the world would meaning nothing to him if he wasnt hitting in the right place at the right time.
(BTW, I found 3rd edition rogues with great weapons to be rediculous. Most of the time people didnt do it because thats the image of the character they wanted, they did it because it caused stupid damage. Yet they would still argue that this was their characters "theme" : good themes NEVER start with numbers)