To my knowledge, all credit cards present their “valid from” and “expiry dates” as numerical dates, e.g. 02/06 – 02/09. So why do many transactional websites request the credit card validity in text date format, e.g. Feb 06? Most people will not know their credit card details – they arrive at the credit card details form, reach into their wallet, select the relevent card and work through the form referring to the card that they have placed near the keyboard. When they reach the date fields, additional cognitive processing is required, translating the number on the card to a month on the form. This interupts the process (do not assume that everybody can make the immediate leap from 02 to February. Spend time observing consumers on the task and it will not be long before you see fingers coming out to help with the counting).
So whilst using textual dates may be considered to be more “user friendly” and understandable, in the context of completing the credit card form, it adds complexity. UI guidelines, patterns, heuristics are all well and good, but ultimately you must consider the context of usage and design accordingly, even if it does at first glance result in something that is not immediately user friendly.