If you are not finding value in having a credit card to earn miles you can cancel, and yes I would suggest them if you don't fly a lot (fastest way of earning miles) and are flexible in terms of booking reward travel (as only a few seats are usually allotted for frequent flyer reward tickets).
Things like magazine subscriptions, online shopping malls, etc. can also give you a chance to earn more. Around holiday periods, online shopping with retailers like amazon and toys'r'us or sears which are affiliated with many of the airlines' reward programs often have double or triple miles in the weeks leading up to the December Holiday period.
