JonBostwick:
No we dont.
We don't know that just because more "loanable funds" exists that more real saving is occurring. All we know is that real savings, that is the creation of capital, increases output. Whether this is good depends on who you ask. Some may prefer more immediate consumption over later consumption.
Sure, government could force people to spend money in ways they don't want to. I'd never call that good.
Well, real saving would be occurring since the 5% you pay in taxes would be invested ("saved") by government. More saving/investment means greater capital accumulation and therefore higher real wages. On the other hand, greater consumption means less capital accumulation, destruction of presently existing capital (as people don't have the funds to fix capital), and a general trend towards lower real wages.
Take a look at hunter-gatherer societies vs. modern society. A hunter-gatherer society consumes almost everything and saves/invests very little, while a modern society with a capital intensive economy saves a lot for upkeep and expansion of capital and therefore of output. To summarize, lots of consumption = less investment = less capital = less output; while more investment = more capital = more output.
Of course you can critique the idea by saying that people's money isn't going where they'd like it to go, but wouldn't that be a small price to pay for "the greater good?"
P.S. I am an anarchist and I'm opposed to government on the basis that I believe in competition among protection agencies, private courts, etc. I would always prefer no government to a government that invests my money for the "greater good," but this tax/invest topic is a very interesting thought I've had recently. In my mind, competition between police forces, courts, etc. would outweigh the benefits of gov't enforced saving (and therefore monopoly police and courts).