76 It actually appears to be legal in all formats, Commander included. Huh.
I guess six mana (including two white) takes long enough to cast that you'll get hit by something in the meantime. And by then, people are probably pumping out damage fast enough that you won't get back up to 40 on Felidar Sovereign's lifelink and vigilance alone. In a dedicated lifegain deck, sure, but then your opponents have other problems.