You have two questions here. Are mixte frames desirable? and what about these two frames.

First: mixte frames can be far less intimidating to ride because they allow mounting without swing one's leg over the seat or top tube. For many, that isn't an issue at all, for some it is at first, others never want to ride a "men's" bike. Also mixte frames make skirts and dresses a lot more feasible. But mixte's have their drawbacks. They are heavier and nowhere near as stiff. So there are real tradeoffs.

Second: those two frames are old French frames. I suspect they are not light or stiff even as mixtes go. They will be French threading and tube diameters making headsets, seatposts and bottom bracket much harder to find. The bottom bracket will also be right hand thread for the right cup, meaning that it will want to unscrew unless tightened very securely. More work for you and the chance that your wife may be stranded some day after the cup loosens and the bearings spill out. The cottered steel cranks on both bikes are a lost technology and very hard to deal with now in 2015. (When everyone had a sorted box of cotter pins and all mechanics knew how to deal with them and had the tools, they worked very well. But any mechanic who came into the field in the past 4o years has had little need to learn those skills.)

So, in short: you can make good but not great bikes out of both of those frames but they will be real work and tougher to work on later.

Ben