Originally Posted by
NeverStop
Imagine this situation. You have 60 RP. Your rune state is 0 Death, 0 Frost, 0 Unholy with 1 Unholy rune coming off CD in 0.5 seconds. If you FS with BT macro'd, you risk BT activating a Unholy rune and putting you at 2 Unholy for 0.5 seconds. In this case, you will have 0.5 seconds where 1 rune is not regen'ing at all. This can happen whenever you BT with 1 rune coming off CD in less than 1 GCD or 1 second.
BT already has a lower throughput than RE, so you can't afford to waste any rune regen by being capped on one type of rune. Proper use of BT would ensure that you only BT when it has no possibility to rune cap you, and within a period of 5 to 10 stacks, you can always find an optimal time to BT if you manage it manually.