We had an extra seat for DD when she was younger which came in handy when it came to DH needing one in his car & also when the grandparents watched her they used the spare one which was always available for them in our garage. We now have a spare booster for DD (safe n sound pegasis deluxe) & we have the safe n sound maxi rider AHR in my car.
No sparie for DS atm but it would have come in handy the other day when PIL were babysitting & we didnt realise they wanted to take the kids to the park. We were running late to be somewhere so didnt have the time to get DS's seat out of my car so MIL stayed home with DS while FIL took DD to the park in her spare seat. So we may look at buying a sparie for DS.
We have 2 bases for our capsule which was great for when he was younger so PIL could use it in their car but now he's in DD's old meridian.

I think if you mainly use the one car for family outings & grandparents etc dont have your child/ren much then one should be perfectly fine but if you're planning on using different cars ALOT then I would get a spare seat.

I hope that made sense!