Caleb's fight with Spike consisted of little more than a single punch while Spike was more or less surprised, and in the process of going game faced. Hardly a demonstration of much beyond sheer brute force. Likewise, his fight with Faith was over too quickly to give any indication of the two opponents' relative speed or strength. Plus, the fact that she might still have been somewhat out of practice fighting demons (certainly not at Buffy's level anymore) combined with recovering from her beatings and near death in Los Angeles might have resulted in Faith not being at the peak of her fighting form yet. A longer, more drawn out fight with either one might very well have ended differently.Caleb fought Spike and Faith pretty effectively. I know that Buffy is faster than both those two in most situations but the way it was depicted was silly.
I wouldn't have minded her using some Aikido- using Caleb's strength against him a couple of time- to thrown him about and keep him off balance until she found what she was looking for.
As for Buffy using some Aikido (or Hapkido, Jujitsu, or Judo) against Caleb, we never saw any indication that she had any sort of formal martial arts training. Almost exclusively throughout the series, her fights were much more street fighting techniques than anything resembling a Japanese, Chinese, or Korean art form. Also, when she had to spar with the Watcher (I forget the episode) in Season 5, she had to "do it her way" to beat him. Most likely, the closest Buffy ever came to martial arts was watching chop sockey movies with Willow and Xander.