On a recent project I had to integrate a front end I built into another environment that had some restrictions. I didn't have full control over the page. It was loading jQuery 1.4.2 and had many elements which required this version. My front end, however, was written for jQuery 1.10.2. It was going to be a nightmare to rewrite all this code the way things were done over 3 years ago.
And we can test that both versions are working. We should have safe access to both versions from the console and any scripts we add to the page.