Here are some of the reasons I think it's a good idea, looking at the very long term!
When you're an adult you have someone to talk to who understands your family's unique perspective on the world.
When you and your partner get old, your DS will have someone to share the burden of looking after you, practically speaking. If your or your partner's health gets bad at any point it'll be much easier for you all if your DS had another DC to help. This is a very real consideration for me. Also on an emotional level it'll help your DS to gave someone to talk things over with.
From your point of view, if you have only one child, when you get old you will have more actual time seeing your kids, as they'll be busy with their own lives but if you're like any of the old people I know time with your kids will mean a lot to you.
Two kids means double the chance of grandkids, again something that can bring real meaning and joy to you when you're elderly.
Also there was research recently showing that if you have a sister it's good for your mental health.
HTH :)
FWIW I have one DS, who's 2.5. DP and I haven't decided whether to have 2 yet, but this year isn't right (we're both studying). A lot of my friends are having their second as they've gone for the 2 year gap thing, but some of them have found it really tough. I don't think a bigger gap is such a bad thing.
How old are you if you don't mind be asking?