Q:

Is there a reason why Nathan uses keycards to grant or limit access to the different rooms in the house?

In Ex Machina Nathan is a genius tech innovator. Does it make sense that he would use keycards to limit access to the different rooms of the house and his computer instead of biometrics or a more secure or advanced method? 

A:

Yes, to avoid the locks being hackable via the network.
A centralized system using biometrics or other methods could potentially be vulnerable to an AI -- or inquisitive visiting programmer -- that got access to the network. The keycards would simply be high-tech keys with access individually coded into each of the locks.
Plus, it's entirely possible Nathan had a less-than-optimal system installed specifically to help with some sort of manipulation of Caleb, since we eventually learn the whole experiment is a setup. 
(From a narrative perspective, forcing Caleb to physically acquire a key card puts him in more peril, and is more visually interesting. As we later learn, stealing the card gave him access to an interface in Nathan's chambers where he was able to revise the security protocols. A fully secure system would have required a more convoluted method for Caleb to turn the tables on Nathan.)

