In reply to Andy S:
If you haven't booked your flights yet, take a look at Lyon for flights plus car overall. Might be cheaper.
If you are definitely staying in Chamonix then a car is not necessary. But, if the weather is bad you have the flexibility - those who think the Alps is Chamonix look away now- to drive somewhere else.
Finally, note that it might still be illegal for an EU national to take a Swiss car into the EU. Check this out on line.