Yes, card companies and other retailers absolutely want to make as much money from you as possible on Valentine's Day - but how is that different from any other day? When the Writer by Night (who has a long-standing hatred of Valentine's Day) and I celebrated our first 14th of Feb together, we made a deal that whatever we did would be free. He made me an awesome comic and I made him a fluffy purple picture frame and a heart-shaped pizza. We still have both of those things (we ate the pizza). You can celebrate without spending anything if you feel that strongly about spending £2 on a card.
And as for the argument that you don't need one day a year to tell you to make a loved one feel special because you should do it every day; well, I'm willing to bet that you don't. I've heard more than once that over-use of the phrase 'I love you' devalues it, but I think that's categorically untrue. Using it when you don't mean it devalues it, using it and then behaving in an unloving way devalues it, but I don't think you can ever say it enough. If you can't remember when you last told the person you love that you love them, you need to say it again.
I am lucky, I know, to have someone to celebrate with and someone worth celebrating. But one of my most memorable Valentine's experiences was a year when I was single and seven friends and I did a 'secret valentine' with a budget of £3 each, pulling a name out of a hat. I got a red fluffy heart that vibrated when you pulled a string (if memory serves me correctly it was from Ash), John won by giving Ceri a bin bag full of inflated balloons and a pin, which when she popped them each had a letter inside spelling out a message. Then we all went to the cinema to indulge Julia's crush on John Simm in 'Wonderland'. That year I also sent someone an anonymous home-made valentine's card and a kinder egg. It was an unrequited crush but it felt good to send it, and I'd like to think that it made him feel a little bit special that someone had gone to all that effort for him.
Love is precious but it's also free and the more you give away, the more you get back. I think it's a sign of a civilised society that there's a day dedicated to celebrating that.