I know that it only applies for the 5th combo. That's why in fflogs you tend to see either WT / FnC is the higher % which means in that fight, you proc the 5th WT / FnC more often. It's still a good DPS buff no matter what. Major fixes will probably come in 4.1 where it adjusts the LOTD mechanic since Yoshi himself tweeted that it does take too long to reach LOTD.

Did an EX Roulette run today and the ST changes made it way better in keeping stacks / able to use Nastrond in AOE pulls which results in a fast run. Not only that, there's this moment in the final boss where I have less than 6 seconds left of BOTD and charge up the gauge back with DS > ST. Really useful.