It is indeed a strange choice to include only two interfaces, and then make each of them different speeds.

Realistically, the wireless side is unlikely to saturate a 1-gig pipe. If the 2.5g is used for WAN because the WAN can use that extra speed, then it's very likely that the LAN port becomes saturated. The configuration never really fits.

The best use I can come up with is this: A 2.5-gig managed switch and PoE with a tiny bit of VLAN magic does allow the OpenWrt One to be an access point+router that can be located in a spot that is good for RF propagation, with only 1 wire connected to it for both power and data. In this configuration, it route packets with up to ~1.25g of WAN (or perhaps more, if WAN is asymmetric).

But even though VLANs are fun and I wish more people knew how to use them effectively, that's so corner case that I'd never rationally expect anybody to actually use it that way. :) And it forces WAN traffic to contend with local WLAN traffic for that shared pipe, which introduces fun new ways for networks to slow down.