Quis ut Deus wrote:Confundor de hac re in sectionem 551c libri Allen et Greenough
Surely that should be "sectione"? I also think you singled out the wrong verb in the second sentence. Instead of "dēfatigābor = future" you should have "percēperō = future perfect".
Anyway, I'm not really qualified to comment, but my guess is that it was just to emphasize that the subjunctive is possible, even though it's rare. A lot of things that are 'rare' are things you will still encounter. I remember a Spanish-teaching website said that the Spanish pronoun "ello" (as opposed to él, ellos, etc.) is "rare", but it's still encountered frequently -- just not nearly as often as the other pronouns.
For what it's worth, Spanish actually requires the subjunctive in similar constructions (except when the subject of the subordinate clause and the main clause are the same; in that case, the infinitive is preferred). The last Latin sentence in Spanish would be, "Antes de que venga, mandará una carta." (But preferred: "Antes de venir...") French and Italian apparently follow the same rules as Spanish here, too. Maybe the use of the subjunctive here is a Vulgar Latin influence or something.