DS is 8 and DD is 4... last time we went he was 6 and she was almost 3, and we did a double. That was nice, but they couldn't keep their hands to themselves and they drove my husband & I insane with constantly having to get on them to leave each other alone. Plus, the double stroller was a bit cumbersome to maneuver, plus the bus, etc. But I wouldn't have done without it. This time, we'e doing 1 single, for DD. We figure DS can hop in every so often if he gets tired.
It truly varies from family to family; not every family does the parks the same way, same length, etc. We have actually never, ever used a stroller at home. But it was definitely helpful for us at the parks, getting quickly from A to B, keeping the kids in close proximity (for better or worse, haha) and to allow a nap if needed.