Although the 331s are to replace the 321/322s, not all 8 of these were used on a daily basis. When all the 333s were available, the 321/322s would operate the Leeds-Doncaster exclusively, with a couple more filling in the Aire / Wharfe triangle services. So I guess in reality 6 331s & 16 333s is (in theory) enough to cover all those diagrams. The same routes were previously covered by 3*321 & 16*333 before the 322s landed, albeit with a few less peak services.
And not forgetting that West Yorkshire was originally to get the 331/0s, some of which would have been operating doubled up on the busiest Aire services. So the situation is fluid.
I believe some have been used for training out of Skipton, but I've not seen any in public service. They all been 331/1s thus far.