The main reason to stockpile blues is that it's an awful big assumption (imo) to expect rank-up mails from before the patch to produce post-patch legendary engrams. Maybe I'll be wrong, but I'm still going to do my ranking up post-patch and get new mails that I am sure will follow the new rules. My inventory isn't 100% full yet, so there's absolutely no reason to take the risk.
I also think people are going to be disappointed in this patch when they find out that legendary engrams have become 3-4x as rare to make up for the guaranteed legendary item. If we're lucky they'll have some inkling about how shitty their loot system is and will only make the engrams twice as rare as they are now.
Honestly though, it barely matters. The raid loot ensures that 25-light armor is obsolete the moment you buy it. It's just a means to get to level 28 to get the real loot, which is on an entirely different (but equally gay for RNG) system. Same with primary weapons, because the elemental damage on them has the same effect as the 30 light cap on armor. Special weapons are all about getting lucky and finding an Ice Breaker, and heavy is just getting a legendary rocket launcher of each element.