Why aproofs of concept considered important? (was Nuke vs. SPS)

--- In spacesettlers@yahoogroups.com, "sailor.barsoom" wrote:
> hitsquad [...] says flatly that X cannot be done for Y reason

What did I say couldn't be done? Please quote me.

> He does weird invalid things such as clinging like grim death to "a quadrillion dollars per gigawatt," and then we find he's just taken ISS and multiplied it.

...As Al Globus did, as well.* What is a "proof of concept"? Why is it considered important?

*
http://alglobus.net/NASAwork/papers/NSSJOURNAL_PathsToSpaceSettlement_2012.pdf
"If SSP's worst critics are to be believed, we must accomplish something close to the following:p
1. Bring the cost of space solar energy collection down to near $1,000/kw, from roughly $750,000/kw today."

"pMost of these items were identified by Steve Fetter in an paper critical of SSP. Fetter's paper can be found at www.publicpolicy.umd.edu/Fetter/2004-P&S-SSP.pdf. This author's rebuttal to Fetter can be found at space.alglobus.net/papers/FetterResponse.html."

>From Fetter's paper: "Today, large space-based arrays cost 500 times more than earth-based arrays per peak kilowatt.7"

"7 The solar arrays for the International Space Station cost about $2.4 million kWp1 ($450 million for about 250 kW or 180 kWp)."