I think part of the factor in keeping it should also be when you plan to have more kids. Are you thinking in the next year or two, or are you thinking 5 years down the road? The length of storage makes a difference.
As far as usefulness...I think it varies by kid, because different kids are....different
My oldest practically lived in the bouncy seat when she was a baby, she LOVED to sit in it. And she was a baby who actually bounced in the bouncy seat. Her first solids feedings were in it also, rather than the high chair. My second, now almost 2, HATED the bouncy. Absolutely hated it. We used it all of 3 times.
The playpen...we have a full sized one, I can't imagine I would use a pack n play much either, because they are small and aren't really useful for containing an older baby...they don't give the baby anyplace to move around or crawl around. But, with the full size, I can still put my almost 2 year old in it and if she doesn't climb out (she climbs everything) she can still sit and play with plent of room, and does at at times when I need to run the vacuum or whatever without her following me around destructing everything I just cleaned
And, like another poster said, when you have a toddler running around, you have to have a safe place to keep the baby contained...or vice versa.
Stroller....I wouldn't be pushing a baby in the stroller while chasin a toddler...rather, wearing a baby while pushing the toddler in the stoller. No chasing necessary then...
If you are talking about ttc soon, I would suggest keeping the stuff for a bit, especiallly if you have a basement to store it in in the mean time. If you aren't ttc for a while, then I don't think it's an issue to just dump it and buy new again.