"Oh, our payment processor is in Node, but our site's in Ruby. Our file caching layer is in Python with bits of C for high performance and our crypto stuff is in C++ with Boost and libcryptopp but it talks to a Java app that uses Cassandra..."
(Runs outside, sets self on fire, shoots self in head...)
Otherwise what would be the point of learning all these languages, frameworks, libs and environments? Of course the learning experience can "make you a better developer", but is it supposed to be only that?
(1) Code reuse, or lack thereof.
(2) Multiplication of surface area for things like security bugs... now you have five different stacks to keep track of security updates for instead of just one.
(3) Multiplication of deployment resources-- now you need multiple VMs, maybe multiple instances with different stacks to run them on, etc.
(4) Developers can't pinch-hit for each other unless they know every damn stack in the world.
(5) If you sell, God help whoever has to support that stuff in your new organization.
It would be interesting to know if an acquisition fell through because due diligence revealed something like this and the purchasing entity didn't want to deal with that mess or it conflicted with their culture.
:-)