You could pledge multiple shares to pump.io (like, 100 or something), which would guarantee the $3 is used up. However, this would require you to send only $3/month to snowdrift; if you had more in the account than that, it might send more than you want.

So, I don't think that's really the way to use it. Instead,
say you are willing to spend $3/month. Also, let's say Evan
says on his snowdrift page that pump.io needs $30k/year. And he does the math for you and says that this means it needs around 1500 shares pledged.

So, you pledge 3 shares. At the start, these provide some tiny amount like $0.01/month to pump.io. That sucks. OTOH, it doesn't cost you much if this doesn't work out.

So you go on pump.io and encourage other people to pledge. Eventually 500 do. Your 3 pledges match them, raising the price/share to $0.50, and so you're sending $1.50/month to pump.io. At this point pump.io is making ~$3-6k/year.

You notice that's still not quite what you wanted to send, so you update to 6 shares and reach your $3/month spending goal.

You also find more ways to spread the word, and eventually 1000 are pledging between 1 and 6 shares. The matching makes each share send $1 now, so you drop back down to 3 shares. Somewhere in there, pump.io meets its funding goal, sustainably.

Certainly seems complicated when all explained like this, but a lot of it happens organically or can be managed by others (Evan, the snowdrift team). And unlike your "just want to send pump.io $3/month" goal, this has encouraged you to spread the word and indirectly contribute a lot more.

With that said, I think a montly spending cap would be a useful addition to their mechanism!