Even though there is no current Mac has more than one internal HDD, but all of them can still connect to an external enclosure which contain more than 1 HDD.
So, I agree that remove the RAID GUI is a huge step backward of the disk utility.
Also, even not for fun, there still a chance that have to manage the RAID array more than once every few years. If something only need to do once over a long period of time should not have a nice GUI, then setup Time Machine also should not have GUI, once it's setup, you will only recover files, but not frequently modify the setting. Or bootcamp assistant should not have GUI, because once Windows is installed, there is no need to touch bootcamp assistance.
In fact, unless you partition your HDD for fun, there's absolutely no reason for it to be GUI-driven. It's not an everyday toy, once the partition is set, it should be last for years as well. Then why Apple still keep this function in the disk utility? They want us to play around with partition for fun?
Also, they should not "waste time" to make a whole new disk utility as well. They can simply use much less time and patch the old one to El Capitan, then most of the user will be happier.