It's not the Sap that aggros anything. It's the fact that your stealth drops, and then you're left standing visible right next to mobs. What do mobs do when you get too close? If you don't have the talent to improve it, you won't stay in stealth when you use it.
This is probably something that only us long time veterans know, because it was changed early in BC, before WoW ever became a mega giant in Wrath. Most players have probably never played when Sap ever did drop stealth, so they don't remember it ever doing so. If you look at the sub numbers and when Sap was revamped, this shouldn't surprise. It's one of those things exclusive to WoW's earlier, much more difficult, days. But originally, it did drop stealth. You had to invest a majority of your points into Sub for a 3/3 talent just to get a 90% chance for it not to. Later, in BC, it became higher up the tree, cheaper, and 100%. Eventually it became free, given to the ability by default. Why was it changed? Well, that's another discussion entirely. But I'll say this much; BC heroics.
Azalus wrote:I haven't played a rogue much, but I'm pretty sure that normal sap takes you out of stealth. So using it in dungeons doesn't work unless it's the last CC used because the rogue will then draw aggro of mobs nearby.
However, there is an improved sap talent that if you have 3/3 gives you a 90% to return to stealth after sapping. That would be necessary to achieve what you're looking for I believe.
You're wrong on 2 different accounts here. Firstly, Sap needs to be the FIRST CC, not last, because it can only work on targets that are not in combat. Every CC by default(No future "no aggro" buffs here) aggros mobs. Once a mob has been aggroed, Sap is useless. It will return an error message if you try to use it. This means Sap needs to be performed first. Other CCs can be used in combat, Sap cannot.
Secondly, Improved Sap did not ever return you to stealth. It kept you from dropping stealth in the first place. These are not the same thing. It means you were never visible to begin with, and they won't hit you or have any time to apply debuffs to you which would immediately drop it if it worked as you suggested. If it did drop stealth, then tried to restealth you, ie Vanish, you could take damage between effects because mobs react instantly, or you could get hit with dots that then just drop the stealth.