Demo bindings have usually only 1 advantage. That the ski now becomes more universal so various size boots will fit. Most have some disadvantages to them also. Most common is the binding though typically is heavier in weight compared to a standard fixed mount binding. Also even though you end up in a size 12 in your new boot the sole length may differ some between brands. They should be close, but your next boots may not be the same. Also many skiers are out there skiing boots that are too big for their foot. Read through some of the boot fitting discussions on shell fitting and proper sizing of ski boots by a knowledgeable shop reps. You can shell fit your existing boots and see how close you are to a good fit. Some of this is a comfort issue (tight boots not well fitted with the right hands could hurt to ski in), but most as they become better skiers and move up to higher end boots will be going down size to a smaller boot. Smaller boot is a tighter fit and more responsive and can be warmer too if done right. If you plan on taking these skis and bindings you are asking about to a shop to mount, discuss with them the issues and concerns. If their response is dramatically different than what you are receiving here- run don't just walk out the door and try another shop. There is usually +/- a few mm in natural adjustment that the shop can do typically before they have to remove the bindings and redrill new holes. If the bindings are mounted for your boot size that you own now, and you do end up getting a pair of boots that are substantually smaller, then you will just have to have the shop see if the bindings would need redrilled at that time. No sense worrying about it now at all- or else get the boots fitted and purchased at the same time. The 2nd mount should decrease the value of the skis for resale slightly (most only recommend 2 or 3 times drilling into a ski for the bindings mounts- but it depends on factors like where the holes fall), and other technical things that the shop tech again would have to call.
If this worries you then maybe you should be looking at some of the system bindings- skis with built in plates that handle a specific binding on the plate and no drilling required. Many brands use this on some of their skis- Atomic, Rossi, Volkl, Elan and probably more manufacturers have this with at least part of their ski line on select models.
Bottom line is pick a shop that you trust, explains things to you, and will do right by you and you should not have any thing to worry about. And spendinga some added time on these forums to gain more knowledge can be valuable too.