Recently used and most used have different utility. Frequently used function works better for items that are used more and more over time, the user needs to identify quickly those items to continue on them an activity routine. For example the start menu in Windows xp listed the application frequently used because programs in a OS art routinely used to get tasks done. Otherwise, "recently used" function works better for items that are used over short period of time. For example, users write documents in Google drive, modify them, read them over and over and then, after weeks, when this document is no more relevant, they skip to another document. So in this case for a user getting quickly his recent-opened documents is a better choice.
Moreover, if a user works on a document routinely, the function "recent used" is slightly transformed in the "most used" since he will often find in the top of the list the sane documents.
Hope this explains