The argument for avoiding promises for promises is fairly simple. A promise is a representation of a value that will exit in the future. If that value is still a promise, you haven't really got to the final value yet. Conceptually having the promise doesn't really provide anything other than a means to obtain the final value, as such it's of no additional use to be able to get the intermediate nested promises: You might as well just jump straight to the value.
This problem is explained in more detail by @erights here. The currently proposed [[Resolve]] algorithm only dictates this recursive flattening for unrecognised thenables, not for promises. THe advantage of this behavior is that a user can pick up a Promises/A+ library and return a deeply nested mess of foreign promises objects (all of which were broken enough not to do any unwrappi