Ahriman
Tyrant
Why? I've got all this money sitting around in the treasury and I'd like to pump it into my research!
Capacity constraints, seems perfectly reasonable.
I see Pazyrk's point, but removing the slider (essentially; fixing it at some constant level, probably 50%) has other problems. It makes you *very* vulnerable to economic difficulties, because you can no longer reduce your science output in order to avoid deficit.
So if you're low on gold, then any economic difficulty can push you into being unable to maintain your army. That kind of knife-edge effect is dangerous.
IRL, a government can use borrowing to smooth cycles, but there's no way to do that in Civ. So there has to be some other continuous variable to let you smooth out economic shocks.
The slider is an elegant way to do that, along with its ability to model deciding whether to spend on your wealth on gold (save it), science or support the arts - a decision certainly made by many rulers.
I guess another way would be to let you go into negative gold, with increasing production penalties and happiness penalties the lower you went, but again you could get trapped in a deathspiral with that kind of mechanic.
[Note for the speculators; I don't remember seeing any sign of sliders in the handful of screenshots we saw.]