It would allow these questions to be easily filtered in or out of searches, for example.
I absolutely think that we should have the proposed [esperante] tag. It will make it easier for contributers who cannot or do not want to respond in English to find the questions written in Esperanto. For example, as a member of the Akademio de Esperanto, I am planning to invite the other members to contribute to the Esperanto StackExchange, as their know-how can lead to very high-quality answers. But as some of them might not feal very comfortable answering in English, it would be great if I could show them the link to the questions tagged as [esperante], so that those who whish to do so can concentrate on answering these questions in Esperanto.
Robert Cartaino argued against the tag based on the fact that it is a meta tag. The link that he claims to contain arguments against meta tags only has arguments against special kinds of meta tags that don't carry over to the proposed [esperante] tag. So no good arguments against this tag have been put forward.
I can appreciate the desire to highlight these posts, but using tags for purposes other than describing the subject of the post is called "meta tagging" and we explicitly try to avoid using tags like that on these sites. You can read about the problems of meta tags and how to spot them in this blog post:
The justification for avoiding meta tags has held up well over the years. In my experience, those reasons are probably more true than ever.
esperante would not say anything about the subject of the question, but rather the language used to write it.
I understand it could be used to filter the questions written in Esperanto, but since there aren't other language Stack Exchange sites using a similar tag, I don't see why we should use it. It is not that difficult to find the questions asked in Esperanto, from the list of questions.