Some invitations have very specific RSVP details such as the date to respond by and by what method: email, telephone call, online response form, etc. Those details are there for a reason.
I have heard it thought that RSVP means to respond only if you are planning to attend and therefore if you don't respond, it means you aren't going to be there. If this is what you thought I'd like to enlighten you to the fact it simply means "please respond" so you do need to reply whatever your response may be.
Do you leave your response until really near to the event date just in case something better comes along? If you really aren't that interested in the event then let the host know with more notice so they can invite others as every event has a guest limit and by holding your place and then not using it you are stopping something else from enjoying the party.
To forget to reply sometimes is OK but to never reply is arrogant. While you may not wish to plan ahead the party host clearly does. This fear of commitment is simply unfair. It is understandable that an event such as a wedding is not as big an event in your life as it is for the happy couple but don't add to their stresses. Sure "anything can happen" is an excuse but in a world of instant gratification we also have to sometimes accept plans for future dates and give notice.
RSVPs are not merely quaint social niceties. It's not an antiquated custom. The host's time, effort and money are involved in planning. Ignoring an RSVP sends a message about how much you value your host so just make a decision and let them know.
I heard someone say recently, "I always thought when you're planning an event you order more food than people invited anyway" which may have some basis in truth but while you may over-cater for a couple of extra guests you don't prepare food for an extra twenty who may show up if it suits them. It astounds me that people can't give the courtesy of a response, yet show up and expect to be fed and entertained.
Don't be afraid to say no. If you're not free or just don't want to go then let the host know. Be polite but don't waste their time. You don't have to give long explanations or justify your decision but do reply.
So come on be honest, do you reply to all invitations you receive?