I was thinking along the same lines. Presumably a softer 2nd serve is always better than a double-fault, because at least you're still in the point. However, the notion of calculated risk in taking a "first serve" as a second serve, sometimes, is interesting. But don't players always calculate that risk? If a player is feeling confident with the serve, and the receiver isn't reading it that well, you might try it. Or at 40-0 or 40-15 you might go for it. Depending on the opponent, and the score, aren't there always times that a player would go for a bigger serve on the 2nd, sometimes?
Sidebar question: Who has the best 2nd serve ace/unreturnable percentage? I'd guess Federer, overall, because he has such great placement, and probably Djokovic this year. Nadal has also worked on placement, so his 2nd serve can be effective. Murray is the one who needs to work on his, of the big 4.