The problem is slightly more difficult than just choosing the location of the wallet file.

The problem is BDB: our wallet file is not just a standalone file, but lives in a Berkleley Database environment, which consists of several files (including log files the database files refer to). Being able to have the wallet be stored in a placed independent from the other databases requires splitting the database environment in two. There are several difficulties with doing this, and it's not a real solution.

The real solution is moving away from BDB for wallets. It was a bad design decision to use BDB for wallets, and has caused us numerous problems already. We've been working on implementing a new wallet format that doesn't depend on a database before, but there have been more urgent issues recently.

In short: yes this needed, yes it will be implemented, but no it's not that easy, and it probably won't be done very soon.