The problem is, let's say SpS gives you an additional cast per 10 GCDs, is that if you're ever put in a situation where you lose that GCD through movement or clipping you've lost any gains that SpS would've given to you sans its affect on dots and auto-attacks. Crit and direct hit are better because they scale better off litany, chain, bard songs, battle voice and spear. They're also better because there's no situation where you're going to get an additional GCD off in a trick attack window with the amount of SpS we're given now, while critting and d-hitting in the TA window gives huge gains; which is amplified to a greater degree during the two and three minute markers when critical/direct hit buffs line up with it.