It all depends on the weather. If you hit a period of rainy days then the temperature gets lower and you feel cold in a municipal alberge or monastery dorm without heating. At this point a warm sleeping bag is indispensable. For the extra kilo or so to carry it is well worth it. I remember a bunch of us sitting outside at a big table, wrapped in blankets as we ate our meal, as the wind was cold, but there was not enough room to sit and eat inside. Also, some people like to open the windows at night which is OK if you have the means to stay warm, but uncomfortable if the blankets fall off you during the night. No intentions of dissing the comment above, but I didn't have a credit card and just used my debit card at ATM's thereby ensuring that the trip stayed within budget. Have a great trip!