Special Items

Sun stone has future uses (2 in gen 5, one in gen 6).

Porygon-2 evolves again to Porygon-Z, which actually has pretty good stats, so keeping upgrades for when that comes out is good (note upgrade did not evolve Porygon-2 to Porygon-Z in main series)

Steelix, Slowking, and Politoed may become much better defenders after the gym rework. Steelix and slowbro are actually good on defense already, but get shaved much too easily. For that reason, metal coat and king's rock have value.

Aside from sun stone, none of these have any direct evolution uses beyond gen 2.

i´d say use some of them because it is a 50 xp bonus for using an evolution item 100xp under lucky egg.

I just deleted a few upgrades but still have three left, which seems like plenty.

I just evolved another 3 Steelix (still don't have IT/HS out of 14 evolutions!!!) and 5 Porygon2's (no HP/HB there either). I also just trashed 6 King's Rocks and 4 Dragon Scales. I'm just hanging onto a few Sun Stones for future gens and Metal Coats in my never-ending mission for that Steelix.