Is extra cabinet depth worth having for equipment I might install in future?

Often, yes.

Rack height is easy to notice because equipment is conveniently measured in U. Depth tends to reveal itself later, usually when the new switch, UPS or appliance you want turns out to be 80mm deeper than anything you originally owned.

A slightly deeper cabinet gives you a wider choice of future equipment and generally more comfortable cable space.

That doesn’t mean everybody should install a 600mm cabinet for a router and one patch panel. Future-proofing is useful; building a small annex around the network is optional.

Where two cabinet depths both fit the intended location comfortably, the deeper one can buy useful flexibility later.