I don't think it is completely true. There are a lot of talents (especially in the T100 row) which are really bad. An example:
Unholy Death Knight have Dark Arbiter and Soul Reaper as their single target options in the T100 talent row. Right now Soul Reaper does around 500 k less dps than Dark Arbiter. This make Soul Reaper really bad and the gap between the talents is way too large.
If Soul Reaper did only did 100-150 k less dps, then Dark Arbiter would still be the choice for the cookie cutter build but because Soul Reaper is easier to use, it could still be picked in fights with very demanding mechanics or for progression even though it doesn't provide the highest damage.