I think having sets that you like to keep together and you don't want to mix up is fine. Too many people saw the Lego Movie and took it to mean that keeping sets as sets is bad. Note that the person you responded to isn't stopping their kids even as they cause more destruction than the Lego Movie showed, they're simply complaining about it here because what _they_ had is gone.
Yes, let kids mix and match and play. But also acknowledge that we all play differently, and for some people having a model of something that they built is where the fun lies. People who like organization can still have fun, let's not shame them for their preferences.
Uhhh... the GP was written in a kinda funny way, so it just seemed fitting to plug a reference to the movie. If you want to keep sets together, power to you. My kids have a mixture of both (sets they want to keep as is and a mountain of pieces from other sets) and... that's just fine.
