Using castrandom doesn't negate the benefits of Haste vs Crit, not sure why you would say that. Haste still makes you cast faster and crit still makes you crit more often, it really doesn't matter whether you have a perfect rotation or not. It might be as optimal, but it still has great effects (obviously, otherwise my DPS wouldn't change with gear).

I use a cast sequence on my DK because I have to really, and because it doesn't really affect anything. On my shamans, using /castsequence drops DPS and causes latency which is why I manually apply FS, manually cast 4x LvB, and then /castrandom the rest of the fight.