In the long run, 75% may not seem like that much, but it would give Venusaur the advantage by a good chunk more then half the time. In this case, the odds are actually on Venusaur's side.Banning all forms of sleep? I think not. However, Spore is a sleep move with 100% accuracy. Almost no other move has the ability to shut down nearly any Pokemon in one turn like that, and the ones that can (OHKO moves) are banned. If Breloom comes in on something that can't outspeed and OHKO it and you have no Espeon/Xatu, something is going to sleep. So, perhaps Spore should be banned, but Venusaur's Sleep Powder has very shaky accuracy and ends up screwing Venusaur over half the time by giving a Heatran or Infernape a free turn to Fire Blast you to hell and back.
Except smogon can't change game mechanics.I think they should lower the accuracy of spore instead of banning it. Lowering it to 80% makes it a lot less safe to use, and so less people may want to use it. If they still use it, like many probably will. They have a chance of losing their Breloom after a miss.
( I use a Breloom with spore btw )