Pass Navigo

I have to rant a moment on the Pass Navigo-Paris's version of NYC's Metro card or London's Oyster Card. After much procrastination I finally dealt with myself and got the Navigo. I was so excited to charge her up and not have to think about those damn little carnets anymore. But how disappointed I was, after several minutes of denial/cursing/frustration when I realized that (mid week and mid-month), you can only charge week (16.80 Euros) or month (55+ Euros)--and this doesn not imply say Wednesday to Wednesday or Mid-Feb to Mid- March, but at the first of each. How can this be? If anyone reads this and can enlighten me I will be grateful. Frankly with this system I will loose money because I don't commute everyday and try to avoid metro's when I can, especially when it is nice out. I want to put 30 Euros on the Navigo and use it for 2 months if I want to. Why is this not the case as it is in NYC and London? Another false trap of French bureaucracy. Tant pis.