On the tax deductibility side, from the perspective of a "DBA", there are only two reasons I buy pre-built RPG content rather than writing my own:
1. Because the subject is complex enough I'm concerned about being able to playtest enough to insure I've achieved proper balance.
2. As research material.
Both reasons are part and parcel to every RPG material purchase I make, as an individual and as a mini-publisher. So, for my purposes, I do deduct all WotC books purchased as research material.
Am I planning to reprint their closed content? No, but that doesn't matter. Every concept in their books is an important part of my inspiration and market research, no matter what form it takes when it gets processed and transformed into an independent idea.
Now, I'm not necessarily advocating that everyone replicate this approach... I'm just saying that, in my case at least, every WotC book purchased does, in fact, contribute significantly to my efforts as a publisher.