There always have been marriage watches but imho there are two sorts of marriages:
1) The "marriages" for affionados like IWC pocket watch movements housed in a wrist watch case, old AS, PUW, Bifora, Glashütte, JLC, IWC movements in a new case in order to save a beautiful movement. Most of them have an "individual worth" to their owners.
2) Bad marriages. Someone is selling let´s say an expensive watch (like IWC, Rolex, Vacheron, Patek) and he does not tell that it is a marriage, e.g.
+ parts are not time identicial (movement from 1975, case from 1960)
+ the movement never has been cased into the model he is offering
+ the dial was made completely new (but not by the brand itself)
+ you get a vintage watch with a brandnew ETA 2824-2
+ movement has been repaired by using not time identical spare parts
I´d always be careful with No. 2, No. 1 could become a collector´s item if there´s a market for it and if the watchmaker got a reputation (Rainer Nienaber, Jörg Schauer, Benzinger to name some).
The Maurice Lacroix with AS 5008 (automatic alarm movement) for example became an interesting item - maybe because the movement was only produced in a quantity of 196.000 pieces.