You're forgetting the issue of context and that there are other results of the caster using his spells besides running through them. I'll give you a random example of how being able to cast two spells a round instead of one can mean a caster actually runs through his spells slower:
1 - Suppose you have a high-level cleric in a group without anyone possessing WRT. During a fight with a BBEG, the cleric uses a spell in the first round and damages the enemy. The enemy acts on his round and (a) drops one or more of the cleric's allies into negatives or (b) poisons them or (c) paralyzes them or (d) does ability damage to them, and so on. In the second round the cleric uses a second spell and kills the BBEG. Then he has to spend a couple of extra spells to restore his allies to full health.
2 - Same situation, but one of the other PCs has WRT. He uses it to allow the cleric to get two spells off, and he kills the BBEG before the latter can damage his allies as in the previous example. Since he doesn't have to use extra spells to heal/restore the allies, the cleric actually ends up using less spells.
And that's just one hypothetical example. Considering the infinite different contexts created by the makeup of the individual group, the type of campaign (WRT allowing spellcasters to cast extra spells will be a whole lot deadlier in a campaign with only one or two encounters a day) and the type of enemies faced, "basic math" is quite irrelevant in determining its effectiveness. IMNSHO, of course.