First of all, to checkout a branch, you don't simply select its name. You need to write a freaking configuration file to tell CC what you want to checkout. And it is entirely possible to accidentally write that configuration file in such a way that you'll have half of repository checked out from one version of code, while the other half is checked out from another version of code.
Then there is history of changes. Or in case of ClearCase: histories of changes. Because there are many, in fact as many as there are files. Yes, each file has its own history of changes, completely independent from other files. So if you make a commit which changes 10 files, CC will create separate commit for each of those 10 files, each with the same commit message. Needless to say, browsing history with same commit messages repeated tens or hundred of times, is quite an adventure.
Oh, and because of that, it is not possible to simply checkout some old version of code. If you are lucky, you will have put labels on all files of needed version of code, so then you can checkout that label. Otherwise the only option is to checkout based on dates of commits, and hope that your provided date won't actually cut any commit in half.