Haste, melee procs, and canni spam on a shaman

[Lunk]

I think I have a vague understanding of how proc rates and haste interact in most cases: if your proc rate is 1 per minute and you swing 10 times per minute you've got a 10% chance to proc on a given swing, but if you then stack enough haste to double your attack rate to 20 times per minute the chance for any individual swing to proc falls to 5%. Close enough so far?

As a 60 shaman, when I'm engaged in a long solo fight I'm almost constantly casting something (like Canni) while also leaving auto attack on for a quick swing in between casts, for a little extra damage and/or useful procs like Tash stick. Attack delay is almost never a factor - I don't care whether the attack is ready to go halfway through the cast or right as I finish. It just occurred to me that having any kind of haste up (spell haste, or a belt with attractive stats like RBG or Spiked Seahorse) might actually be detrimental in that situation, cutting the proc rate on individual swings while not really increasing the number of swings I'd take 95% of the time. Am I wrong on the mechanics, or should I stop lusting after a Spiked Seahorse Hide Belt?