This is the reason, I think about reservations in slots as minimum as 10%.
This idea help on reducing selling pressure on bitcoin market if weekly payout from campaigns reaches exchanges on regular basis and in significant percentage.
Also, this may help on reducing spam in this forum because people who treat campaigns as income source may move on.
This idea doesn't make sense at all even by your logic,
Let's say 100K dollars in Bitcoin as payment for every active campaigns combined and 10% of it would be 10K and now the daily trading volume of Bitcoin is in billions, do you think this will really make any dent in the price?
Holders is not equal to low spam cause those are two different things.
And forum doesn't dictate rules of signature campaigns, it's the managers job at the best interest of the brand they are advertising.