Don't know if my opinion would be contrary...
4 AA's or a rechargeable battery half the weight, I know what I would prefer. From my own experience when playing with the DL2 and K100D, unless your batteries are fully charged you get significant AF and shutter lag. From people on this forum I was told that even NiMH AA's don't give optimum performance speed-wise and that the CP-E3's were the best bet of which the only rechargeable ones are hard to find and Pentax didn't advise using them.
Sorry, I would prefer proprietry batteries any time. Consistent performance from fast charging and supported rechargeable batteries, that is what a pro needs more than the availability in the field of AA's in the stores which slow the camera down.
Of course I write this as a pro who will easily use 1000 frames shooting a wedding and needs to know that he has fully charged batteries always at hand. For the hobbyist or someone not shooting that much, lithium batteries will give the performance needed though they are still not rechargeable. I think Pentax is welcoming the pro market with this camera and is therefore having to work the way pros are working, rechargeable, small and consistently performing proprietry batteries.
As for the PC Sync. There are very few radio slaves which screw into a hotshoe. That means that you can't rely on the slave not to partially slip out and not fire. This happens often to me to the point that when working with radio slaves i sit them on the hotshoe and plug it into the PC socket. When you are shooting groups fast at a wedding or during a model studio session you need to be able to rely on your equipment performing, not having to check the screen every single shot. If you are using a PC sync cord from studio strobes then having it plugged into an adaptor on the hotshoe is a royal pain in the neck compared to plugging it into the side. There is a reason why the most modern DSLR's have a PC port, pros use them the whole time.
Apart from that and very importantly for me, I need to both use a TTL flash and fire my radio slaves at the same time, I use this setup for all my weddings, the radio slave plugged into the PC sync fires the strobes to light up the background, the TTL flash lights up the foreground. Because of the preflash you have to use a radio slave, optical slaves won't work. I know several wedding photographers using this system, it isn't an obscure niche...
As far as TTL flash, I've been fine with the ETTL II on my canon and so has most everyone using either ETTL II or i_TTL both of which use preflash, that is tens of thousands of photographers using either canon or nikon who are working with a preflash. Even when using wireless ETTL with uses multiple preflashes, the amount of closed eyes caused by the preflash is exactly the same ratio as when I'm using manual strobes in the studio, in other words it is not a significant issue in the real world otherwise pro photographers wouldn't be using it, they would be using auto flash as they did in the days of the truly awful original ETTL.
Anyway, for me at least, 2 out of 3 ain't bad...
I was seriously considering changing from my Canon 5D and system to two K10D's for my wedding/event business, I'm buying one anyway with the 3 pancakes for a street rig, one of the only things holding me back assuming the files are good, is that damn PC socket! Otherwise, damn, the CF's on that K10D easily rival the 1Ds I used to own, talk about being able to customise a camera, WOW!
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