It's a nice thought but one that isn't done cooking. Lexus is well above Acura in prestige, however Lexus has two faces. Should Lexus tackle the ES or the IS? What about the GS and LX? If Buick tackles the IS, where does that leave Cadillac?
Lexus is having trouble gaining performance cred to compete with BMW. Nissan....*cough*... Infiniti has better performance cred because they dropped all the floaty models. Sure the IS has all the makings of a BMW buster, but Lexus is still fielding vehicles like the ES, RX, and LS. Lexus doesn't have a firm definition because it's vehicles are all over the map.
There is no problem having both Cadillac and Buick in the same price tier. Cadillac goes more euro-sport and Buick goes more traditional luxury. It makes the brands much more focused than the muddle that is going on over at Lexus. GM is in a unique position by having all of these brands.... if they succeed at tightening each brand's definition there is no reason at all that Saab, Pontiac, Buick, GMC, Cadillac, Saab, and Hummer couldn't all occupy the same showroom. By doing so, they'll make Toyota/Scion/Lexus even more "just a Toyota".
The definitions are there... we can all see them... we just have to squint a little...