Some time ago, I was present at a discussion of a bidding problem
(2S) X (3S) X;
We understood that 4D is the normal bid. We were contemplating the sanity of 5D. I contend that the Ace in the opponents' suit is a serious negative feature of this hand.
To see why, lets look at the power of an Ace
1) Controlling power :An Ace controls the suit. It is a quick trick and a sure trick. The Ace controls the transportation between all four hands. Aces exert maximum controlling power at no-trumps when either side has a long, strong suit.
At a trump contract, their controlling power is often diminished because our little trumps can usually do the same job.
2) Developing power : An Ace complements other holdings within the same suit. Aces help build tricks. Qxx in one hand is practically useless but combined with Axx it appreciates in value to half-a-trick. Similarly J109 starts deriving enormous power if the opposite hand contains an ace.
True appreciation of the Ace comes not from the one trick it takes but the additional half-tricks and full-tricks it adds to other holdings. An ace in the opponents' suit at a trump contract is unlikely to contribute any developing power.
I was highly influenced by a series of phenomenal articles Grant Baze wrote under the heading "Baze on bridge" on OKBridge about ten years ago. I could only find this link.
Partner's hand on the problem posted above was
Notice the difference in the playing potential of the two combined hands if we transform the Ace of spades into the Ace of clubs.