Well, the way I see it (and I'm often wrong
). When you are trying to defend someone's keep/guardian you are pretty much proclaiming to be with the group, and maybe even die for the group. In that case, then join them, or their "affiliated" groups (keepers/hammer, druid/guardian, etc).
However, what I was alluding to is that most CRS defence from non-affiliated friends can be seen as a slight bending of the rules. Sure, you can RP a reason, but it can come down to mechanics. Less actual members logged in means more NPCs to help defend. So, if you have one logged member, and five "buddies" that aren't in any cabal/tribunal, you have the army of NPCs, plus your buddies that the system doesn't take into account to calculate how many NPCs you get.
Then there's the possible second reason. You already have a char in another group, and for whatever reason you wan't to play this alt and be able to help defend, and thus possibly kill and get loot from the raiders, who (this is streching it probably too far) just coincidentally might be ppl you don't like with your main but can't do much about it (i.e. members of the same group).