According to quite a large number of discussions here, X-dressing is pretty much separated from a sexual fantazy for many x-dressers,
so the comparison with "playing baby" which is a sexual fantazy, is not valid, I think. Unless, one is into some kind of exhuberant fetishist transvestism, which would become a major sexual fantazy for which your comparison to wearing baby diapers would hold.
The main difference with a sexual fantazy is that being dressed as a woman, just makes the x-dresser look like an ordinary woman, providing "she" is sufficiently skilled at finding proper clothes and correct makeup style.
For x-dressers who tend to dress like ordinary women, they would go unnoticed in the crowd. While an adult (any gender) wearing diapers and baby clothes would certainly not go unnoticed in any crowd.
However, I get your point. What would be our reaction if our SO came out to us with a very different life style, as different from the female "norm" as our X-dressing "en femme" is from the male "norm"? I don't have a proper example of such a "change", but that would make me think a lot.
Actually, the closest comparison would be if my SO would tell me that all these years she was a FtM X-dresser and didn't dare to tell me. But even then, for this comparison to be valid, I would have not to be myself a MtF X-dresser otherwise I would be biased in my judgement.
So on the principle, I agree with your questionning: We, as X-dressers should try more often to put ourselves into our SO's shoes... But I don't think the comparison with wearoing baby diapers works.
Eugenie