like Gonzales posted, waiting to block means you're waiting to be thrown.
but the other thing is, even if the opponent doesn't throw you, they could just attack you with an unanticipated move. I mean, everyone has at least 15 or so ground attacks, so I highly doubt you'd be able to memorize all of those, for all characters (21 in all, do the math). and everyone has a bunch of moves that are fast enough that you simply can't react to.
so yeah, you're either going to be hit by something you didn't anticipate, so your SB timing will be off, or you'll get thrown.