> A problem we face where I work is how designers sometimes limit themselves in the creative part of the work, because they don't know how to implement it later on.

My experience is actually the opposite. If the designer I work with is thinking about HTML (for example) he limits himself where implementation might be difficult. (I do the same in my own design.) So by separating the roles completely, there's much more freedom – the designer designs without thinking about implementation, which forces me to upskill, which means he can push design further next time, in a virtuous circle.