In my opinion any 'advantage' of an immediate pregnancy can also be a 'disadvantage'in the right situation. For example:
- Your kids will be close together in age - advantage - a built in playmate.
However, they can't share a crib/toddler bed, which means you have to have 2 of everything, costing more money - disadvantage (unless you co-sleep). When they're older they can share bunk beds (assuming same sex kids), etc.
- Getting through the 'potty training' period quicker - advantage.
However, training 2 kids at nearly the same time can be....frustrating - disadvantage.
- Related to #2 - getting through the 'terrible 2's quicker - advantage.
Having 2 kids go through that period at the same time... disadvantage.
- If Mom is breastfeeding, she can continue with the baby - advantage.
However, trying to juggle breastfeeding 2 babies at once: a newborn that's clusterfeeding every half hour and a 10 month old that is hungry and/or wants to comfort nurse - disadvantage if you can't get a good feeding schedule going.
As far as time between pregnancies; in my opinion there should be at least 9 months between them. Pregnancy is very taxing on a woman's body and back to back pregnancies even more so. The uterus needs time to contract, the same with the muscles and ligaments which have stretched to support the stomach.