Is > ever necessary?

You don't need to absolutely because almost any XML interpreter will understand what you mean. But still you use a special character without any protection if you do so.

XML is all about semantic, and this is not really semantic compliant.

About your update, you forgot this part :

The right angle bracket (>) may be represented using the string " > ", and must, for compatibility, be escaped using either " > " or a character reference when it appears in the string " ]]> " in content, when that string is not marking the end of a CDATA section.

The use case given in the documentation is more about something like this :

<xmlmarkup>]]></xmlmarkup>

Here the ]]> part could be a problem with old SGML parsers, so it must be escaped into = ]]> for compatibilities reasons.

I used one not 19 hours ago to pass a strict xml validator. Another case is when you use them actually in html/xml content text (rather than attributes), like this: <.

Sure, a lax parser will accept most anything you throw at it, but if you're ever worried about XSS, < is your friend.

Update: Here's an example where you need to escape > in Firefox:

<?xml version="1.0" encoding="utf-8" ?><test>    ]]></test>

Granted, it still isn't an example of having to escape a lone >.

Not so much as an author of (x)html documents, but more as a user of sloppy written comments fields in websites, that "offer" you to insert html.

I mean if you do your site the right way, you wouldn't hardcode your content anyway, right? So your call to htmlentities or whatever (long time no see, php) would take care of replacing special characters for you.So sure, you wouldn't manually type > but I hope you take measures so > is automatically replaced.