Being a purely mental action means you can do it while paralyzed, which is nice. It's generally a lot cheaper to spend an extra 2nd-level spell slot than to use a slot 1 level higher and spend a feat on Still Spell. If you use Silent Spell so that Whispercast is also purely mental, you can cast while all but paralyzed (you still need to be able to whisper, which probably isn't waived with Silent Spell). With no verbal or somatic components, a whispercast spell can't be identified with Spellcraft as its being cast, so opposing spellcasters can't counterspell it without randomly guessing. It completely removes cheap material components from the equation, so you can save on Eschew Materials if your DM is the type to steal your component pouch all the time. It's situational, but it has its uses, no doubt.