Just open them all. Split them between Nia and Rex however you want but even if you get plain Blades you can use them for merc missions. Also like someone else pointed out, even the plain ones have skills (Botany, Water Mastery, etc.) that help with other things in the game like opening treasure, doors, accessing secret areas, doing side quests, etc.
Even if the skill level is higher than any one Blade (for example. Lock Picking Lv. 4) you can use any combination of lower skill levels to hit the 4....so 1, 1, 1, 1 or 1 and 3 or 2 and 2, etc.
Also as you raise your merc levels you gain access to more missions and can increase how many Blades you have in reserve. I'm about to hit Level 4 in merc missions.
Fred.