@stefan
Despite RFC 4193 selection algorithm for ULA ensures low risk of collision, ULA registry (if maintained) ensures no collision can appear.
As ULAs are not publically routable but can be routed in private organizations, avoiding a prefix already registered prevents conflicts when merging networks/organizations.
My ISP delegates me a fixed /48 prefix but I'm using an ULA prefix that simplifies network management (DNS) and ensures ipv6 connectivity stays up even when ISP prefix delegation fails (had issues few months ago with the DHCPv6 client of my previous router).