I am kinda split on this change, since I kinda dislike the errata Merchant Ship and (especially) Patron got. Is there a reason why the playability conditions aren't "when you can play a Treasure, you can spend Coffers". I know I can't outthink Donald X when it comes to Dominion design, so he must have a good reason for this.
That said, is there any consideration on making Treasures playable during the entirety of the Buy phase (kinda contradicting what I just said above)? This mattered pretty much never pre-Renaissance, but nowadays it feels kinda ugly.
EDIT: I have thought about it and yes, above suggestion is also less than ideal as Gamble suggests that you can play a Treasure. So a more ideal solution would be adding the convention that "A Coffers is a $1 producing Spoils that is always available". But that might be a too much of a breeding ground for rule questions (such as: do they count towards Storyteller and Voyage? Can I Mint them? etc.). And that probably sums up why Donald chose his solution instead.