Sometimes we think that a tag should not exist when we see an excerpt/wiki edit when someone tries to establish the tag for the first time. At the same time, should we reject such an edit when it clearly improves the tag excerpt/wiki, or should we temporarily accept it until we eliminate/discuss the tag?
No, tag wiki edits shouldn't be rejected for this reason
A good tag wiki is one matter; whether the tag should exist or not is another. The matters simply don't relate. “I don't think this tag should exist” is just not a valid reason to reject an edit to its wiki. Review the tag wiki edit based on the edit's own merits. Assume the tag will stay even if you think it should be gone and give us a good wiki to work with while it's here.
If you don't think a tag should exist, the appropriate course of action is to open a question on meta suggesting its removal. Having a good tag wiki helps us manage this process by helping us understand what the tag really is and isn't about, so you should be approving good tag wiki edits even if you think the tag shouldn't exist. It also means if we actually decide to keep the tag it has a good wiki—you help the site run smoothly even if it's not entirely the way you'd have it work.
Declining the edit because you think the tag should be gone, on the other hand, is disruptive. It's withholding the opportunity for the site to work just a little bit better, even if the tag is truly doomed and everyone will agree with you to remove it.
It also means that when it comes time to ask us to remove the tag you'll have withheld information from us: you set the tag up for failure and left us not fully informed. Someone may notice that people were attempting to make that tag work better and others were obstructing that effort. This will hurt the case for removal and sway people to keep it around for further improvement and testing.
If you really want the tag gone, you should be steel manning it: that's representing the best possible version of the thing you oppose (the opposite of the straw man). You can then show that even given the best case it should still be removed. That's not done by voluntarily making the tag function worse by withholding otherwise valid edits to its wiki.
If you have a question about a specific tag, you should ask about it specifically.
To me, this question seems like it is pretty difficult, maybe impossible, to give a really good answer to that isn’t “depends on the tag”. So that’s why I’m going to offer: it depends on the tag and the context.
The better approach is, I think, to just discuss concerns we have about individual tags and individual tag curation actions when we have them, rather than try to shoehorn several unique scenarios into a general discussion like this.