It's the carbon/plastic fouling on tubes that particularly irks me. All my 682 combos were very prone to it.
My KX-5 not nearly so much, but the tube *will* build up fouling over time. The IM tube that came with it shoots straight with a good full pattern so that one stays in all the time except for cleaning the damn thing. If I could have gotten fixed full though, I would have. (I still have it, BTW, and am keeping it).
Then I got a used K-80 logo gun with 32" O/U M32 fixed choke barrels. (First thing I did was have KI rebuild the internals and install double releases). Then I added a 34" M32 top single, fixed full, of course. No more screwing around with tubes.
So yeah, I prefer fixed chokes. In theory, the ability to change a choke tube and change the pattern sounds like a big advantage, particularly to novices. But hey, we're talking trap here. To mind, the other disadvantages outweigh that single supposed advantage. And some folks waste way too much time and thought (and money!) on them.