one benefit of the CDP-100 is that it has real MIDI ports instead of USB
Yes, this isn't really an advantage; however, if it were, the Kawai ES110 comes with midi ports, a better action and better quality sounds. Real midi ports was thought so much of an advantage, that Roland didn't even bother to include it in their FP-30. I'm not sure what advantage you get having real midi ports, except maybe the ability to interface with old music equipment.
Yes, ES110 has MIDI ports, as do many other boards, but ES110 is more than triple the price he can get the CDP-100 for.
As I mentioned, the advantage is for gigging. It's a more professional feature than you typically find on low-end pianos these days. It is rare to find a high end portable keyboard without it.
Gigging musicians very often have setups that include 2 (or possibly even more) boards. With MIDI ports, they can be connected together, which can provide numerous benefits, from layering of sounds, to being able to change all your sounds simultaneously from just one of the boards (depending on the features of the board), to being able to play any sound at your disposal from your choice of action/key range.
For example, I have a Nord Electro E5D which is a great board for organ, but it also has great piano sounds. I usually play its piano sounds from an attached Casio 88.The combination of the two boards is very lightweight, and I have all the sounds in my rig playing from the kind of action I prefer for that sound.
ETA: Gigging musicians also sometime bring other gear (besides other keyboards), that also gets connected via standard MIDI ports. A lot of it may be older, as you suggest, but there are new pieces as well, like the Roland Integra7, or their Boutique synthesizer modules like the JP-08. And don't dismiss "old music equipment"... older gear can still be very useful, and can often be picked up used at good prices, too.