She could just use grief/outer layers of grief that isn't holding such things. Not really a reason.
Wait, are we still talking about why we don't want to just release two Witches worth of Grief out of our control right now? Okay, here's a few:
1. If that Grief disperses, the most likely thing that's going to happen is it turns into two newly-hatched Witches or a horde of Familiars. Right now we're
in the middle of a public park.
2. Madoka and Madoka's dad are
right there in front of us. Experimenting on releasing tons of Grief while Homura's waifu and future father in law are potentially inside the radius of effect will make Homura
frown at us, maybe even
pout. We're talking Sad Fluttershy levels of existential angst here, people.
3. Nagisa is a few minutes away, and we're supposed to be having a relaxing picnic. Neither of those things is terribly compatible with running around for the next hour or two fixing whatever catastrophic failure mode will ensue from two Witches worth of Grief suddenly getting free from our control.
4. Even if nothing catastrophic happens, we're still talking about releasing into the surrounding environment enough solidified human suffering that, if it were applied directly to Soul Gems, would probably be enough to cause the entire magical girl population of Earth to instantly Witch out. I cannot
imagine that there will be no negative consequences to doing such a thing.