Alexban,
That is not using your final reserve fuel, it is committing and using your alternate fuel, so that you land at destination with no less than final reserve fuel.
Think of the scenario where you have been given an EAT at a single runway which eats 5 mins into your alternate fuel. You can either:
1) Stay at your destination with "landing assured" and land with (alternate fuel - 5mins) + final reserve fuel or..
2) Divert to alternate field (single runway) and now definitely landing with Final reserve fuel.
The argument that oft comes up is that what happens when you commit to destination and the runway gets blocked.......well I put the theory that what happens if you divert and when on approach with final reserves the runway gets blocked!