Actually, I was facing the same dilemma past few days.
I wanted a mega-zoom lens, with constant aperture of the zoom range and WR feature.
Of course, not such lens exist.
So you have compromise a bit here and there.
Advantage of an all-round zoom is
1. Does not require much lens-changing, which is VERY important on walkabouts and when you have your kids along
2. Reduces (but not eliminate) dust exposure to your sensor with the lowered frequency of lens changing.
But, cons is,
1. Photos will not be as spectacular compared to a more expensive long range prime (i.e. DA* 300) or fast zoom (i.e. DA* 50-135 or Tamron 70-200 f2.8), though some will argue that this is subjective.
My case, I already have a DAL 18-55 (red kit for my wife's KX that I share with my K-5) and a DAL 50-200.
Wanted something that I don't have to keep reaching into my backpack to change lens.
But having the same 'difficult' requirements as I wrote above.
In the end, I decided now to get a 18-55WR and get lens pouch to attach to my holster or belt to store the 50-200 so its easier and faster to reach. Saves me money for now and the is combo pretty much covers my needs (18-55 covers daily walkabouts I do and 50-200 for that extra zoom and nature walks on weekends).
Having said that, if you feel that the $300~ price top-up for 50mm zoom difference is justified, then the Tamron 18-250 is heavily recommended.
Otherwise, the DA 18-135WR is just damn sweet.
Near silent SDM motor (tried it out at SLR Revo and impressed a Canon-toting friend into seriously considering a Pentax K-5 next year) and WR with reasonable zoom range is the next best option, although the price is way above the other alternatives.
Just my noob-ish $0.02 worth of opinions...