You're thoroughly underestimating the power of Snapcaster Mage...though to be fair I kind of did at first also. Sure, Snapcaster removes an Instant/Sorcery card from your graveyard, which does slightly depower Runechanter's Pike, but the fact you can cast it again kind of makes up for that (not to mention you don't always have a Runechanter's Pike in the first place). And even ignoring its flashback ability, a 2/1 with Flash for 1U is not a bad deal.
The lack of Instant speed instead of Sorcery is still killer, though, in my opinion. I just don't think it's worth it to depower the card just for a fairly expensive flashback cost.Enh...not really. The Mono-Blue build that took 4th at the most recent SCG Open (here) didn't play any outside of 1 in its sideboard. Now to be fair it's a different deck than what we're talking about (it's not quite mono-Blue either, due to the Restoration Angels in the sideboard), but it's clearly not as vital as Snapcaster Mage.I've found Silent Departure to be pretty good(though its not Vapor Snag), but its certainly better than Unsummon in any aggro deck where your not looking for a tempo gain but just a way to bounce blockers. Its very overshadowed by Snapcaster however, which does everything it does, and more, and adds a 2/1 flash body as well. That being said, it's not good for the Mono-U Pike deck for the same reason Snapcaster isn't, as you don't want to be removing Instants and Sorceries from your graveyard.
The highest-placing deck that looks to be exactly what we're talking about (that is, actually mono-Blue) is this one, which took 25th (which still isn't bad out of 400+) and played zero Cavern of Souls.
Even assuming Cavern of Souls might be worth playing, as a 4 of it makes no sense. While the fact it can make creatures uncounterable is nice, don't forget the other big reason it gets played in U/W Delver: Mana fixing for Geists and Restoration Angels. That means that a mono-Blue Delver deck has less of a reason to play it, so it makes no sense to play with more of them.