Some loose thoughts...
1) I concur with Dave Metsky (big surprise) on the "need" issue. I think one thing that needs to be considered in increasing
all of your gear's ability to function in deep cold. That is, all of you gear taken collectively dictates how cold you can go with comfort and margin for error, not just the boots. Once you know where that temperature limit is for you and your gear, you can operate within that safely, even if you don't have plastic boots. I simply won't go on overnights now if the forecast is for temp below -10f and I've been quite content to cause partner's upset at cancelling or delaying trips until the deep freeze lets up.
2) I can go to about -15f with a combination of VB socks, leather hiking boots and insulated gaiters. YMMV.
3) IMO, the big benifit of liners in boots is that you possibly eliminate the need for carrying camp booties at night if the liners are designed with laces. (Some modern warmer thermo-moldable liners don't have them.)
4) The big benifit of plastic shells is keeping wet snow out from melting through your boots.
5) The secret to keeping your boots dry on overnight trips (double or single boots) is to use VB socks during the day. I use single leather boots on winter climbing and skiing trips but don't get frozen boots generally.
Here are more thoughts on VBs:
http://home.comcast.net/~pinnah/DirtbagPinner/vb.txt
6) For skiing, the primary trade-off you will need to wrestle with is the height of the cuff. For skiing on steep rugged terrain, most folks want as tall of a boot as they can get. But for walking on uneven ground, most folks prefer lower and softer cuffs. For AT type of gear, the progression goes something like this: plastic climbing boots, low cut AT boots (dynafit, scrapa f1), mid height AT boots (scarpa laser), full blown AT boots (Lowa struk????a, scarpa denali), alpine ski boots. There is a similar progression for nordic/telemark boots. I've not seen many folks using lace up climbing boots on AT skis in the Whites. Based on my experience with nordic and tele gear, I would think this has to do with the fact that approaches in the Whites are usually short (argues for higher boots) and the terrain is steep and tightly wooded (argues for more boot). The folks I've known who like climbing boots on AT skis have almost all been from places like the pacific northwest where it is possible to high mountain trips on glaciers while avoiding really steep and gnarly skiing. There, the skis are used as sliding snowshoes and more emphasis is placed on better climbing/hiking ability of the boot as opposed to its skiing capability. SUMMARY: Unless you have something specifically in mind, you might do better to keep your hiking and skiing boots separate.