I just did a test drive after downloading 3.2.1 and copied the files to the htdocs folder of XAMPP 1.7.4 without any issues.
It loaded again without issues in my browser and displayed the main screen after I entered my MySQL root user credentials.
I created a database and tables without issues except it was not too helpful for beginners - navigation was a bit clunky without a menu system or back button facility and no defaults settings to help along the way.
Creating a new user was not helpful for beginners as you are presented with a ream of tick boxes that would mean very little unless you were familiar with what they all referred to - no warnings about giving a new user full privileges which should only be reserved for super user root and the help was a link to the MySQL web site after clicking on the Privileges title above the check boxes and again not very helpful for beginners to get a handle on granting database privileges as MySQL are notorious for deeming all users have a university degree in reading gobbledygook.
In conclusion I would prefer that phpMyAdmin be the default GUI for MySQL in XAMPP or this forum would be filled by new users asking for help with understanding what this Adminer was all about.
phpMyAdmin is quite helpful in many ways for new users which might only be my opinion having been using it for many years but having said that I did give Adminer a good workout using my accumulated experience and it does what it is intended to do.
I doubt it will get the nod from the developers either but you can do what I did and add it manually to XAMPP so those who want an alternative can use it without having it become part of the XAMPP installation and seemingly there is no advantage in having it integrated in XAMPP anyway.
Thanks though for bringing it to the attention of this forum and, if you have not already done so, you could post in the German language forum but remember you must use German not English in those forums and or the phpMyAdmin forum which is multilingual.
Thank You for test driving, well I am a new user to mysql, php So phpmyadmin is more comfortable than adminer; I found Adminer and wanted that someone Professional like you test it and gives his/hers opinion and maybe somehow this discovery helps to develop better XAMPP, who knows!
Anyway, Thank You for your time and Knowledge Sharley I'd better continue my php learning I'm on lesson 2 btw, I refuse to post on German forum, I don't know German language.