Remember what I said, One Cube, for me it's more of a price issue. But then I know which ones I like neat and which one's I'd rather mix with and then I buy accordingly. And I'm talking about a general rule of thumb. If all I had was say a bottle of WT101 at $20 and I wanted a whiskey sour, I'd make it with the WT101!
But your point is perfectly reasonable. And I would agree with you that if I had a $30 bottle that I simply hated, I'd probably mix with it too...while drinking a cheaper one neat. But I'd bet you wouldn't knowingly buy that $30 bottle again just for a mixer (or $20 MM!). In fact, like you, I think a double duty is really a good way to go. My EW7 and Beam White are double duty pours. I don't like 'em as much as my KC, WT, or EC12, but they're a nice change now and then.
There is no right or wrong, really. It's what makes you happy.