Right, and I like the ability to re-build a release from 6 months ago and get something that reproduces the behaviour of the previous build - i'm not concerned

Right, and I like the ability to re-build a release from 6 months ago and get something that reproduces the behaviour of the previous build - i'm not concerned if the binary is identical, more that it is functionally identical. A typical way of breaking this would be for build scripts to move forward but not be made backward compatible, or easy for the previous version to be used for an old build (for example, a newer compiler getting used rather than the one originally used). I've worked on enough stuff where releases get held up for various reasons (and so production software can be 6 months behind head) and there's a desire for a fix to the released version and hence changes made to a branch as well as head to apply the fix. This sort of thing is made much easier if you don't have to spend time trying to work out how to get the branch to still build!