Is FortiLink so hard to use?

There are many ways to configure FortiSwitch through the FortiGate with FortiLink.  Your DMZ, WAN or LAN interfaces are not wasted when using FortiLink but it depends on what you're looking to accomplish.

The first thing I'll start by saying is as of FortiOS 5.6, you can configure multiple FortiLink interfaces on the FortiGate.  Refer to the FortiSwitch Devices Managed by FortiOS 5.6.3 guide on the Docs website and on page 61 you'll find the following information:

Enable multiple FortiLink interfaces

NOTE: Only the first FortiLink interface has GUI support.

Use the following command to enable or disable multiple FortiLink interfaces:

config switch-controller global

  set allow-multiple-interfaces {enable | disable }

end

Regardless if you enable multiple FortiLink interfaces or not, you can still use multiple physical ports in your single FortiLink configuration; it just depends on the interface type you have configured.  If you leave the interface in Hardware Switch mode with multiple physical ports selected and set the entire segment as Dedicated to FortiLink, you can then connect multiple FortiSwitch devices directly into the FortiGate.  You can also configure multiple physical ports on the FortiGate into an 802.3ad Aggregate interface allowing you to leverage LACP with active-passive or active-active failover to multiple distribution or access layer switches.

If I were to argue, which I try not to do, I would challenge your requirement on needing interfaces directly labeled as WAN, DMZ or even LAN on the FortiGate.  If you're leveraging VLANs across your network, you could just as easily create WAN, DMZ and LAN VLANs and connect your devices into your switches without needing specially labeled ports on the FortiGate.  Starting at the FortiGate 200E, these interfaces aren't identified anyhow and every port is simply labeled port1 through port16.