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Taro,
Well said.  I agree that UZR is not an unreasonable single defensive point to touch on when attempting to get a feel for good/bad defenders.  My objection to UZR is that I typically see it being wielded as an exact, unquestioned, single-stat trump card in many player discussions.  (Not necessarily that you are one of these less reasonable people).
But, UZR, (IMHO), has gotten to the point, not only is it being routinely utilized to judge this defender as good/bad or better/worse than another -- but (and this is the part I feel is actually making analysis WORSE) -- that the specific runs saved numbers are being espoused as gospel. 
Per UZR Beltre saved 15 (ish) runs per year, (except 2007, when he cost 3).  So, people start using that 15-ish runs saved as if it is a reasonable expectation of how many runs he'll save every season, in any park.  ERA measures what happened, so does OPS.  UZR is a mixed stat, partly what happened, partly what might happen in a perfect world. 
My reservations with UZR are largely due to the blind reliance on the "expected out".  In 2009, Figgins and Felix made 314 and 312 assists, (109/110 POs), in 1339 and 1342 innings.  In the same games, they made nearly identical plays in almost every way imaginable.  Raw stats - equal.  RF/9 - equal.  FP - equal.  But UZR pegs Figgins at 16.7 runs saved, while Felix is at 5.3.  Why?  Because according to UZR, Figgins had 249 expected outs, while Felix had 305. 
Ibanez in 2007 gets 224 POs for Seattle - in 2009, 213 for the Phillies, (nearly identical outs in nearly identical innings).  In Seattle, he gets a -20.5 UZR - costing the Ms 20 runs, in Philly he saved 8.  Why?  Because, in Seattle his expected outs were 248, while only 202 in Philly. 
Can I swear that Felix didn't actually wave an extra 50 grounders by -- or know with certainty that Ibanez really did let an extra 50 balls drop in Safeco, (a park notorious for holding balls up)?  No.  Which is my point.  I don't know the reality in that regard.  I also don't know how to divide credit between pitcher/defender.  But I do know fielders make 20 outs and pitchers generate 7 in modern baseball.  I do know that with a perfectly average defense, pitchers are responsible for 100% of runs saved/lost.  I know that with a perfectly average pitching staff, that defenders are 100% responsible for all runs saved/lost.
I personally believe that the actual "runs prevented" assumptions for UZR are wrong - and that the reality is that each hit prevented (beyond the norm) is roughly equal to 1 run prevented, (though I'd settle for 0.8 to avoid the need to shove exponents into the equation).