You can do everything with Linux, but RouterOS gives it to you in such nice user/admin friendly package. It comes with some limitations, but mostly it’s worth it. It’s only natural to want to push the limits a little further and get the same comfort for more services. The question whether something belongs to RouterOS or not, now that even whole network monitoring system is included… can you really say that DNS server does not belong? 
I see no problem with few extensions to what we have now. The basics are already there, it does not require much effort to add the rest. I myself would like to see two:
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Per-domain forwarding. When you have some internal domain handled by your server, but don’t want to depend on it completely for DNS, i.e. if it happens to go down, you still want to be able to use public internet. You could use router as DNS cache, use ISP’s resolvers by default, but define an exception that queries for company.lan should be send to your server. A proper alternative to old L7 hack.
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Allow to set static records of any type. Few most popular can have specific WinBox dialogs/CLI options. The rest can be supported using generic syntax (DNS server does not need to understand all records it serves, you can sometimes see unknown types in current DNS cache and it’s no problem at all). You can stop here and it will make many people happy.
But as bonus (I’m not actually asking for this, just saying that it’s nothing complicated), you can add a simple option to send replies for selected domains marked as authoritative and you have proper DNS server, even though it would be just a very basic one. Add AXFR transfers for master/slave operation and you’ll start to get out of scope of simple extensions. No, what I wanted to write was that you’d have everything you need for basic server, if you don’t require anything special like DNSSEC.
Real full-featured DNS server is a different story. And there’s also difference between authoritative server and resolver. Authoritative server with all bells and whistles is not needed by most people. Resolver could be more interesting, e.g. if you want to validate DNSSEC by yourself in the name of security (you might not trust ISP or fear evil hackers between ISP and you), it would be nice thing to have. You can get dedicated machine, but why couldn’t your shiny new $3000 CCR handle such relatively simple task.
But as I wrote before, simple extensions first. MikroTik can save full-featured DNS server for later, when they fix all bugs, add all more important features, dominate the router world and have nothing better to do.