It's a great round!! I have a 25-06AI that is within 75-100fps of 257Wea. factory ammo so I don't really have an application for one, the 2 are a coin toss for my uses. One gun can do it all, depending on what you hunt, & I did the one caliber thing with 3 calibers in the past, but I now really prefer smaller guns for PD type Varmits & bigger guns for Elk & above. As stated, I did that trick 3 times in the past, first with a 30-06 Groundhogs & upwith 150 gr, then 7 Mag with a 115gr. Speer for varmits & 160gr. Sierra for Big Game & then the 25-06 std. with 100gr all the time & then 115gr or 117 gr all the time. All would work, but not the best at the 2 extremes.
But fatercat I know what you mean, the 257 comes about as close to ideal for all for one round as it gets, if that what a person desires.
However, many miss the 264WM, a logical alternative & arguably slightly better. It has been overlooked, & not as many good loads can be found, but is undergoing a resurgance due to the great bullets. When you look at a 264 vs a 257Wea, as W. Van Z. pointed out years ago, the 264 mirrors a 257 very close on the light end through 125gr in trajectory & velocity & kicks butt at long rang with the 140gr. Accubond or esp. the Berger. For example, the 100 BT in both are very close, the traj. of the 115BT & the 120BT in 6.5 are dang near identical, but the 140 gives the 264 a real edge for big animals, long range flight path & wind bucking. I have one on the way.