I dont play fiddle , but DO play a lot of mandolin , and dont see ANY problem in a serious player being able to do basic stuff in ANY key . I accept that if you are doing a paying gig you dont want to be missing the odd note , but I enjoy having a quiet busk along in all sorts of keys at a couple of places I go to . In fact I REALLY like a couple of things in F and Bflat !
C# , F# , and BMaj are a bit of a struggle though !