doesn't make sense to not show the expiration on the node teaser and full node

The key-word in your request is 'option'. This would also require some sort of permissions integration I think. Consider the following couple of use cases:

case1: A classified ads site where people pay some sort of subscription/fee so their ad can stay published for a certain period. The way I see it...

- The site admin(s) might want to see the expiration date (or a countdown) for each node. I say *might*, because they can always create a view to view nodes (in mass) sorted on date expiration. I don't see a need for them browsing the site node-by-node though (in which case they'd need this feature).
- I also see a reason for the ad owner to see this date too, but only for *their* ads. Its obvious because they need to renew.
- I don't see a reason for site visitors to know this expiration date though.

So, user permissions is the only way I can imagine we'll be able to achieve this.

case2: An eshop/auction site. Again sellers and site admins need to see the expiration date, but this time site visitors (potential buyers) need to know for how long each node will be available too.

Finally, imagine a site that features both classified ads and auctions. In this case we'd need a per content type setting to enable this feature or not, combined with permissions for each user (for each role actually).

Please let us know if you had a different real use case scenario on your mind when filing this.

This is a feature request + new features go to the latest dev. Also, this doesn't belong to the UI (don't mistake the UI with what the site visitors see). I'd like to see this feature realized as well, but I personally believe this would require quite some coding in order to be implemented. So, the module maintainer(s) might want to set it to postponed, but we should leave this up to them.

...

...a view to view nodes...

should make better sense to say

...a view to display nodes...

;)