My experience of two (wonderful) CMs over the past 3+ years is as follows:
If the CM is available to care for my children then I pay even if I opt (for any reason) not to send one/both.
This has happened if one/both is ill, has been ill with anything contagious (and would also include D+V but neither has ever had it!) Is also the case if either I or DP has a day off work and opts to keep children home to play!
If CM is not available due to either her own sickness or that of her DC then I do not pay. In this situation DP has to take emergency leave to cover as we have no other option. (I am a teacher and emergency leave is very hard to get!)
Regarding holidays - CM closes for 5 or 6 weeks a year and does not charge for these times as she is unavailable to care for my children.
When I take holidays (for 13 weeks a year as my DDs do not go at all during school holidays) I pay a 50% retainer to hold my place for after the holidays.
I do not object to paying the 50% retainer but I would think VERY hard about paying someone when they were not available to care for my children thus requiring me to find alternative emergency care.
These have been the contract details with both my CMs and they have both been marvellous (and current one still is!) However, we knew the t&c's going into the contracts so I would think, had you agreed to pay then you either need to pay or to re-negotiate the contract.
I would not be able, personally, to leave my DDs with someone I resented paying - we pay a huge whack of my salary as it is and feel we get our money's worth and more.