Here in Arizona, alfalfa is the best quality, most available, and cheapest hay (not saying it's cheap, it's about $16 a 100 lb bale right now, but even that is $2 cheaper than bermuda hay, our second most available hay). Sure, I would love to feed a grass mix, but sometimes it's cost prohibitive or just flat out poor quality. Who wants to pay $18+ for 100 lbs of cruddy quality hay?
Needless to say I am currently feeding straight alfalfa. Beautiful, lush, leafy green alfalfa. The adult horses do fantastic on it! The 3 yr old also does fantastic on it other than being a little more energetic than I would like. But hey, he's a 3 yr old and my friends constantly remind me that I am not used to riding a young horse (he's my first).
But anyway, EVERYONE I know out here feeds an at least 50% alfalfa diet to their horses, most 100%. I refuse to believe a handful of cubes or pellets would make someone's horses nutso. Furthermore, I have an ex-broodmare who is now my trail riding horse. I think I have seen her in season 3-4 times in the 4 years I have had her. Definitely NOT mareish.
I think alfalfa is everyone's favorite scapegoat!
Man, if grass chills them out THAT much, I wouldn't know what to think!