fppmongo

That's false. I'll show it formally via a proof by contradiction.

Suppose a room at a Mercure costs precisely 40 EUR. I have two options to pay for it:

1.) Using 2000 Accor points.

2.) By paying with 40 EUR in cash.

Your claim is that 2000 Accor points are worth 40 EUR. That is, your claim is that options 1. and 2. are equal. An equivalent way of expressing your claim is that one Accor point is worth 40 EUR / 2000 = 0.02 EUR.

Now, by Accor's ToC, I won't receive any status or rewards points if I choose option 1. But if I use option 2., I receive 100 Accor points (at the very least: in practice, I get more when I have status or when I can use an extra-points promo).
But 100 Accor points are worth 100 * 0.02 EUR = 2 EUR. So, if I use option 2., my total cost is 40 EUR - 2 EUR = 38 EUR. However, this is a contradiction. Option 1. isn't equal to option 2. after all!

Further introspection will reveal that an Accor point must be worth strictly less than 0.02 EUR.

EDIT:

Or, explained differently, the correct way to represent the cost of option 2. is:

40 EUR - 100 Accor points.

When paying with cash, you effectively pay less than the cash amount because you receive Accor points in return which are equivalent to some positive cash value.