Yes because obviously the Thalmor are gonna welcome her with open arms at their embassy.

People are capable of looking younger than they actually are, you know. She was MEANT to be a "pretty face", as Kematu puts it.

That's not even addressing the issue Skyrim has of inconstant ages, so much so that someone made http://www.nexusmods.com/skyrim/mods/19730.

I've just done the quest again, and I always side with Saadia. I've probably already read hundreds of lines of text about this issue... be it on wiki talk-pages, archived talk-pages or various forumposts all over the internet.

Anyway, here some of my thoughts which can be applied even if both don't say the (full) truth:

Saadia pulls a dagger. But this can be explained by her being scared, on the run and maybe slightly paranoid due to the situation. However, she apologizes quickly after she notices you're not gonna harm her. The reason for her being in Whiterun might be that WR is a neutral city and might neither get taken over by xenophobic Nords, nor the Empire which would bring Thalmor with them. Or maybe she's there because she just didn't get any farther before the Alik'r arrived and started to harass Redguard women on the streets. Then the guy in the prison... he claims that Kematu will kill you on sight. Why would he think that if they were officials? Then as I already mentioned, they are harassing every Redguard woman they see on the streets. Sounds more like mercenaries to me, than like soldiers/guards/whatever with an official mission. Next is Kematu's attitude towards you. He belittles and taunts you, and sounds quite arrogant overall. That's not the way he should talk to me if he wants to convince me that he's the good guy. The fact that they catch her alive if you side with him just means that they are required to do so (reason could be a trial, but also torture or a husband wanting his wive back alive for personal pleasure and/or to keep abusing her or whatever), or they don't want you to know that they are lying, because you know... you could be quite dangerous. After all, you've killed all those bandits in the cave; so you might be able to kill them. Which might be another reason for him talking to you instead of attacking immediately in the cave btw. ^^

Anyway; those are some of my reasons for siding with Saadia. Okay, and I also like that she calls my male characters "eye candy" haha... but I would ignore that if I'd think she's evil! Oh, and... I've read in some places, that apparently you can find a burial urn with her name on it in the Hall of the Dead if you side with Kematu. I do NOT know if that's true, but I've seen it written a couple of times. IF it is true, it would be a quite strong hint towards Kematu lying, as he claims to bring her back to Hammerfell for trial.

It DOES happen sometimes, but it is a bug.