XLRs used for speakers was very common in the past. very few if any cabs are made with XLRs these days but i still see some companies replace speakons with XLRs to match their existing setups and there are still plenty of old boxes out there that exclusively use XLRs, old rigs in pubs and clubs are full of them and other strange connectors that will really make you go
. Speakon is fairly new and its only in the last few years that its been adopted so widely. Jacks were never reliable in large rigs because they do not lock in.
There is nothing rare or bizarre about their use, its what was around so its what was used. Some of the larger multi-pin plugs and wiring it all is a lot to get your head around though.