Our DD9 and DD8 build per instructions, then play with whatever they just built. I posted previously about DD9's obsession with not wanting to take her creations apart...ever! We have been lucky enough to purchase some bulk used Legos (just a large tub full of odds and ends). For some reason it is okay to build and rebuild with these particular Legos (sometimes), but if it came in a box, with specific directions, then in DD9's opinion - that is what it was meant to be.
The girls received several Lego Friends sets for Christmas and have put them all together now. They have an entire town set up in a corner of our front room floor. Certain pets go with certain people and they take their pets to the vet, the park, and to the stable, etc. They go to the store and everyone has a job. They only have 2 Lego cars, so some Lego families have to walk. Eventually some of their older creations will find their place in this new town and some things will morph into other things, but there is no taking things apart without causing major drama.
Teehee - sounds like a LEGO SIMS game. I like it!!
My older son (now 7) is only now starting to build with instructions. He prefers free form building always, although he got a BIG set that he asked for for his bday and is slowly but surely working on that.
My middle son (5) prefers to build on instructions only and then keep what he has built. He got the Star wars X-wing set for his birthday and works on it a bit each day.
My little one loves his duplo and prefers for now to build his own way - mostly large towers and what he calls cranes.
The only structure that remains intact is the giant VW Bug, but that was really for me.