no there ARE reasons to avoid maxing, but if you want to know your max there's one iron clad way to get it. None of your perfectly good reasons (and yes several do exist) have been stated in here, and thus my method is no better or worse than yours. What concerns me is how many people come through here wanting a way to get that number without actually finding it (threads like I can do x weight for y reps over z sets, how strong am I?). then somebody gives them a formula and they think that's it, that's their max, and they go around telling people that number they came up with. It fucks things up for people who actually get under the bar and want reliable, first hand numbers (like in the sherdog's strongest man thread).
All in all, you are right, and your formula does have merit. I personally am benefiting more from just moving something progressively heavier until I can't.