Because, and I know you'll agree with me here, there are plenty of people around who can't ignore OOC information and like to use it ICly.
Definitely. Add to that that many people can't handle evil stuff that is directed against them very well or at all.
However what does that mean we should be doing? Does it mean that we should make the system support secrecy so that they just don't know (until it's too late, that is, meaning that there will be quite a lot of OOC trouble and OOC whining and so on when they find out), or does that mean that we should consider very well whom to pick in order to play with (meaning that the number of potential RP partners is severely decreased)?
The alt spying has become regarded as something bad and sneaky. However, such secret-rank-spying would be very similar, and lead to the same issues eventually.
I think that you cannot force people to be able to RP well. It doesn't even work with RPing something universally agreeable. How much worse will it be if we start trying to force upon them something that they can't even handle OOC-ly? Especially people with less than average RP experience / ability are unable to separate IC and OOC, which is the reason for this thread. That means they will be unable to distinguish evil RP from being jerks OOC-ly. Worse even, the reverse will also be true.
Playing evil will lead to OOC trouble, be it justified or not. The system cannot help there. The only thing that this would IMO do is to facilitate claiming "That's just how the game works, so shut up". That's a very easy argument to make, as it doesn't require consideration of the particular instance (and therefore is a "quick fix"), but doesn't create welcoming atmosphere.
I'd rather see secret rank spying than alt-spying, though (since it's less OOC), but I still don't think it solves the actual problem of people who just can't deal with it. BTW, secret guilds have not solved this, either.
Regarding the "tarnishing of the guild's name" I can only say that a guild that doesn't manage to screen their recruits decently deserves no better. Also, if such a member creates trouble, the guild leader is the one to talk to. However, this is not possible if one is deprived of the ability to acquire that information.