Many times people complain that WordPress isn’t ‘complete’ because it doesn’t have the features they feel are indispensable. While explaining, and educating, to people that they can’t always get what they want out of core is often fruitless, the point remains that WordPress cannot, and should not, do everything. That’s a part of a longer post, but once you are willing to accept the fact that WordPress will never be everything to everyone, we can move forward.
There are practical reasons why Multisite doesn’t do everything people envision, and there are also some weird impractical reasons. Sometimes the answer to this is a very simple one, sometimes it’s confusing, and sometimes it’s technical. But there are reasons, even though we don’t like them, and over time they change or go away as issues are resolved.
Here’s a list of the common questions I get and their answers.
Why can’t I manage everything about all sites from the network admin?
It would slow your site down. You’ve probably heard me say something like “WordPress Multisite was intended to be multiple separate sites managed by one install of the WordPress files.” When I say that, I don’t mean ‘managed’ the way some people envision managed. When I say ‘managed’ what I mean is “You manage your network settings from the network dashboard, and you manage your per-site settings from the per-site dashboard.”
Not every site has the same settings, anyway. The point being, if WP had to query every site for every setting (like all your comments), it would get really really slow when you have 11k comments on your blogs. And you’d be mad.
Why can’t I set all defaults from the network?
Oh you want them to be all the same? I’m not sure you should be using Multisite, but since you are, this one is annoying.
In normal WordPress, you shove everything into wp_options and call it a day. On Multisite you have an options table per site, so wp_2_options and so on, but you also have wp_sitemeta. Some settings and plugins have been coded in a way to fall back, so it grabs defaults from that table. But. Not everything has been, because there’s a tacit assumption they don’t need to be.
Now there are plugins that will let you force in defaults on new site creation, but given that every single site admin can go in and change them, it would defeat the point. They’ll just change them! Thankfully, it’s a growing trend for plugins to include a Network Admin Settings page, where you can determine how to control things for your network. But that’s only going to be for things like caching and probably Jetpack. Not ecommerce. Yet.
Why can’t I share media?
Because media is content and content is per-site.
There’s a plugin called Network Shared Media which purports to being able to do this, but by it’s nature, content is sacrosanct and separate.
Why are usernames using characters like – . _ are not allowed?
Because they’re not permitted in subdomain URLs and since the blog URL is generated from your login name, it’s not allowed. The site admin can do it from the back end, though. This is highly imperfect, and perhaps should be changed (so maybe we could also have less than four characters in a name, which not even an admin can fix). The logic to do the detection would be a pill though, and you’d still want to do it on things like a mapped domain.
Why can’t I just map a domain!?
That’s being worked on. Eventually WP will have that superpower.
Actually that brings me to the biggest reason why Multisite can’t… It’s not that it can’t, it’s that it probably shouldn’t, or it will be able to do so eventually. If you consider what the original intent of Multisite was, much becomes clear. Open Source Development doesn’t move at your pace, not even if you’re Matt Mullenweg. It moves at the pace of everyone involved in the coding and testing, and it moves at the pace of their ability to innovate. That means sometimes what you want isn’t there yet because it’s not easy, or because it’s not ready, or maybe because no one agrees with you right now that it’s a priority.
After all, sometimes we’re wrong about Open Source Developmen.
When I tell people over and over again that they 
I don’t care if you use it or if you use multiple separate sites. I do care that you understand that multisite is complex and I care a great deal that you’ve researched your options. I care that you ask questions like “If my database gets too big, can I split it with Multisite?” (Yes, use HyperDB or SharDB.) I care that you consider “Is a custom post type slower or faster than a separate site on a network?” (Neither, as it happens. They’re about the same.) I get really happy if you ask specific questions, actually. You know, the ones that tell us you’ve looked into this and thought about it.
Meanwhile another store sells cows. All the cows live at Persephone, and are shipped out to Jiangyin and other stores. They’re doing well with everyone going to cowship.com, but they too want to have jiangyin.cowship.com and so on. Each store lists what cows are for sale that live well in each location because Holsteins’ don’t like Bellerophon, who knew? This way, someone on Jiangyin can order Holsteins but not Texas Longhorns. Obviously they need to control which product can be sold at each location, same as the Dollhouse, but they also have a different problem. Their product amounts must also be stored in one location and shipped out from there, so they want to make sure they don’t oversell their cows.