I've used two for extra slots now as the combination of weapons perks and strike packages mean you must have the correct loadout not only for the map but the game mode and your play style too.
I have one left and I don't see the point in using it for double weapon xp as that'll just be reset once you've prestiged
Two hours of double XP ok if you want to win the prestige race but obviously it's gone in two hours and not coming back
The emblems - meh, there's plenty to choose from anyway
I'll be back upto 2 once I've prestiged and I'll see what the next unlocked options do exactly. If the unlock gear option keeps my M4A1 unlocked throughout the game I may well do that so I've always got a weapon with all attachments and proficiencies to fall back on should I find I need one for any reason.
I'll probably go with another custom class for the last one.