The gap between my DC is 3 yrs and 1 month. WE did have a double for a short time - bought second hand on eBay and sold a year later for £7 less than what I aid for it. Like zzzz it was a P&T tandem (Sport). The reasons were very similar to zzzzz's:
I walk a lot, it keeps me happy and keeps me fit. Its 2 miles to the next village, and I used to of this several times a week for playgroups, shops, etc . No way would 3 yo DS been up for this.
DS was, and still is, very tall. Too tall for a Buggy Board to have been practice, especially as the buggy we had had a single "loop" handle, rather than 2 single ones with space in-between.
It's very hilly where we live, which is also difficult for a buggy board.
DS still napped when DD was born, and I was so not up to being housebound by his naps (or indeed hers).
DD was born in October, and the double was invaluable for getting DS to nursery that cold wet winter!
Sometimes, when the baby is crying and the preschooler is tantruming, you just have to get out....