I decided to work some numbers for Leech Seed, so here it is!
Compare these two Celebi:
EVs: 252 HP / 64 Def / 160 SpDef / 32 Spd
Bold Nature (+Def, -Atk)
This has 404 HP, 277 Defense, and 276 Special Defense. This gives it an overall defense stat of 404 / ( 1 / 277 + 1 / 276) = ~55,850
EVs: 226 Def / 252 SpDef / 32 Spd
Bold Nature (+Def, -Atk)
This has 341 HP, 321 Def, and 299 Special Defense. This gives it an overall defense stat of 341 / (1 / 321 + 1 / 299) = ~52,780
This is a difference of only 5.8%. However, against a Pokemon with 404 HP, Celebi recovers 50 HP, which is 12.4% HP regained for the first set, compared to 14.7% for the second. This means that, if Celebi wishes to Subseed, it loses 9 HP every turn on the second spread, but 26 for the second. The first one will run out of Substitute PP, whereas the second will only be able to do so for 12 turns, assuming it starts out at max HP.
However, imagine a Pokemon like Sceptile, who is far more likely to be Subseeding. It usually has a 252 SpAtk, 252 Spd spread. Compare the differences in Subseeding for a Sceptile with a 31 HP IV and 4 HP EVs and a 3 HP IV (for max power HP Ice) 0 HP EVs Sceptile.
282 HP, 165 Def, 206 SpDef (17 HP from Leftovers, 70 HP per Substitute)
253 HP, 165 Def, 206 SpDef (15 HP from Leftovers, 63 HP per Substitute)
That's a 10% decrease in defensive abilities. However, the HP of a Pokemon needed to Subseed against for all 16 PP of Substitute is 312 for the first set, but it's 288 for the second spread. In a Sandstorm, against a Pokemon with 404 HP, the first set can safely Substitute for 9 turns, the second one can do so for 13 turns.