Another post already. I just wanted to get something out that it seems the general populus doesn't get.
In the grand scheme of life, it may not matter much... but in wedding planning, it's a big deal.
That card and return envelope aren't packaged in the invitation just for looks. The couple and/or wedding planner would sincerely like for you to fill it out and return it. It doesn't take long and it saves a lot of grief if you do it. It provides something tangible for whoever is planning to see that you are attending and it's the same little card everyone else (in a perfect world) sent back, too. Whether or not you are able to attend, it's a courteous gesture to return the response card - even if you're not 100% sure you can make it. It may be better - at least in my case - to go ahead and say you'll be there, then call back later if your plans fell through. It's more stress to say you're not able to make it, have them plan for you not being able to make it, then show up at the wedding when no one is expecting your bright face there.
Knowing who and how many guests are coming to the wedding is a big part of the planning process, directly tying into the financial aspect. This is especially true if the couple is planning an affair that charges per guest. This may include using the venue and more likely if they are having dinner catered and served. Ordering too much is a waste and not ordering enough is ridiculous.
If there is going to be some sort of assigned seating, they'll need to know who is coming so they can seat people who don't clash by each other. You surely don't want to sit by Uncle Bob and Aunt Janet, do you?... Or how about Grandma Agnes? Remember how much you hate when she pinches your cheeks? And you're 35?
Even if you don't want to pay the million dollar postage that's holding you back, a thoughtful phone call or e-mail would be more appreciated than no response at all. The last thing the bride wants to think is that these people she invited to share in her special day want nothing to do with her, hate her or her husband-to-be, think she smells, ....
I, and every bride out there, would kindly like to ask you to respondez s'il vous plait.
Edit: I forgot to address the whole plus one issue. Receiving an invitation does not automatically entitle you to bring along a date or friend. Again, it ties directly into finances. Whoever is paying for this wedding may not have endless pockets to pay for every guest and that guest's guest. It is fair enough to reason that they budget for who received invites. Typically, family and bridal party can bring guests. The bride and groom would also typically be considerate enough of people who are in serious, involved relationships to not leave out the significant other. Everyone else, like Flaky Jane who has one week flings, is not necessarily going to be "allowed" to bring a guest.
One sure-fire way to be sure you can bring a guest is if you receive an invitation addressed to you "and guest". Another way is if you're unsure whether or not you're allowed to bring a guest... ask! It's better to ask than just assume you are!