Quite true, it may be the way to serve as a temporary fix. I just think deeper about this issue.
The watch is mainly to receive things, sometimes the user may reply on the watch, but should not use it as the basic way to send things out.
So, since both phone will log in to the same Apple ID, receiving msg will not be an issue. All notification base on Apple ID (e.g. Reminder) should also works without any problem. Emails also not a problem.
Forward cards can also fix the SIM card issue.
5s is good enough to provide other functions e.g. maps or fitness training.
Therefore, most of the functions actually works (or being extended). Of course, it is not perfect. E.g. You still can't remote control your 6+ camera via the watch (which is a much better camera then the 5s). And the watch may not able to receive all instant msg (some apps do not allow multiple login, if I prefer to use that app in the 6+ because of it had a bigger screen, then it won't work on the watch). But it really depends on the user why (s)he want the watch and how (s)he use it.
For basic functions like checking email, receiving calls, sending out the drawing / heart beat. This work around actually works very well.
So, to sum up.
Pros: most functions will work, have an extra iPhone as backup, able to pair up the watch with a JB 6+ indirectly.
Cons: extra SIM card required, cost more money, not all functions on the 6+ is extended, required to carry an extra iPhone to "pair up" the 6+ and watch, the watch is rely on the device that has less battery life.