Why do people care about feat taxes?

[Divine Susuryu]

RAI seems to be that if for some reason you no longer qualify for an ability, you can't use it.

That can't be universally true in the case of PrCs, because then we wind up with Schrödinger's Dragon Disciple, who by disqualifying himself with the half-dragon template loses the class feature that grants it, hence re-qualifying as a non-dragon, gaining the template, disqualifying, etc.. So either that's the intended rules interaction (no it isn't) or you can lose a pre-requisite after entering and still be fine. Now, this is contradicted by CArc and CW, but erratas state that the primary source wins out in contradictions - the primary source on PrCs being the DMG. The DMG received reprintings of errata after the publishing of those two books and nowhere in that errata did it include text to that effect. So, if the CArc and CW rules are to apply anywhere, they are to apply only to PrCs they contain per the primary source rule. This is an old argument, and it's pretty definitive if you follow the chain of logic.

It is, and I can say this with 100% certainty given the evidence above, a house rule to say otherwise. RAW indicates that for PrCs outside of those two books, you do not lose abilities for losing prerequisites, as there is no rule in the DMG to flow to those other books nor is such a rule in those books. RAI is a bit more complicated, but I contend that since the rule appears in those two books, and not ones printed before or after, the intent was also that prerequisites were for entry only - after all, if that were not the intent, they would have reprinted the rule or errata'd it in to the DMG. They did not do so.

Is it a good house rule? Depends, but for the majority of groups, its about as meaningful as a law against sprinting faster than 10,000km an hour - it will just never come up. For the groups where it matters, it'll either be a very good house rule or a mildly annoying one.

But if you run with that argument, by that we should weigh the likelihood of a DM allowing Pun-Pun as a vanilla level 6 Ranger. No, it's one thing to say that certain mechanics exist within 3.5, it's another entirely to say that all mechanics are equally likely to be allowed in an actual gaming group.

ICYMI I did say that in an at-the-table discussion rather than a RAW theory-crafting discussion it's different. I still don't think it's hugely helpful in an at-the-table discussion when compared to actually knowing what the specific GM in question would or wouldn't allow, but it does have value there. What gets my goat is that in RAW discussions people bring in GM fiat - it's just another variant of the Oberoni fallacy there.