Why would you say it's useless for re-hydrating weed? I can't speak for Boveda, but I've re-hydrated weed with Integra Boost packs before very easily. I agree that it can be done the way you said, and never said it couldn't, it's just "cleaner" to use a sterile pack and you also don't risk adding other flavors to your weed. I used the orange peel method when I was younger and could always taste the terpenes from the orange peel in my weed.
Why..experience that's the reason. You'll have to quantify how much boveda you need to raise how much hydration x how much weed but overall my experience shows me it's too much task if the weed is overdry. Without exception.
An orange can mold but you can see it and it only molds if left in past it's useful purpose. Re-hydrate and remove,
then add a boveda for maintenance.
Celery does not mold, it just turns yellow and that's after a long while. I recharge my boveda packs in a quart jar with a stick of celery. Done in about 12-16 hours, no smell, no water, no stains, no damage to the exterior of the pack. I've done it many other ways and that is the easiest method.
Not saying it's impossible to re-hydrate with Boveda if you use enough of it, just that it's fairly useless and inefficient when compare with either an orange peel or celery stalk. Both of which will get the job done faster. Why take longer to fix the problem by using Bovedas for something they were not meant for. Bovedas maintain humidity to an extent. They are not meant for re-hydrating.