I'll throw in my own little bit of experience...
First, I believe that any of the La Crosse and similar chargers can "test" the batteries just fine on their own, without the need of a separate tester. Just put it in Charge/Test mode and you'll see exactly how many mAh are being pumped into the batteries.
What I've done is numbered all my NiMH batteries individually with Sharpie markers (which unfortunately seem to wear off over time, I need to find a better solution), then use my La Crosse BC-900 to test them all and mark the results down on the grid. I can now look up any battery on a big list and see exactly how many mAh it should have when fully charged, and weed out the really weak batteries.
The Refresh mode may help, too - with the charger, I got four La Crosse 2400 mAh batteries, one of which only took 1616 mAh, after a refresh, it took 2410 then 2510 next time I tested it.
Most of my batteries are Lenmar 2500mAh ones (got a 20-pack from buy.com for $21 a couple years ago), these all saw many recharges in a cheap one-hour Rayovac charger but most are still OK, but the weak ones are 1800-1900 mAh and the good ones are 2300-2500mAh.
Now, back to the Pentax content. I have an *ist DL and a K100D, and have found myself with no usable batteries more times than I care to remember. Just a couple days ago, I was trying to fire up the DL (I had ground down an old Ricoh-mount lens and I could get it to mount on the DL but not the K100D), and I tried three different sets of batteries - all of which had been charged within the past week or so - and all of them gave the empty batteries warning on both the DL and the K100D. I tossed a set onto the charger, and each battery took about 250-300mAh to be fully charged, and at that point, the cameras were happy with them and showed full charge.
Furthermore, these batteries generally work great on a vacation setting or similar, when the camera recieves heavy usage and the batteries are charged nightly. I can get around 350 shots per set, maybe more.
So, from what I can see, even if the battery still has 2,000 or so mAh left it in, that may not be enough to satisfy the camera. Perhaps the voltage output drops when it's not fresh off the charger? I don't know, but if I just had one camera, I'd probably suspect a problem with the camera, but since both of mine are behaving that way, I'm going to guess that it's just the way it is.
Either way, I'm keeping my eyes open for a deal on Eneloops or similar low-discharge, I'm tired of being frustrated when I want to use the camera and can't find any batteries that work!