They are useful until you get a character to ~30, then the money to exp ratio is to bad. You can still use them is you don't know what to do with your money.
I'm swimming in gold so I still use them on the higher leveled knights, but if you're not... level up 2-3*s in preparation for the Nation-specific events that I've heard of (where you can only use knights from a specific nation), I guess. Or sell them if you're desperate for inventory space without using gold.
not sure if i should ask in the same post but which is the best place to farm manyus
sg? or the manyu daily
if manyu daily is it better to do 80 stamina one or 50 stamina one considering i can clear both
Tough call, depends.
Do you ONLY need/want to focus on Red/Blue or Purple/Yellow? Feel free to smash up the dailies, but be prepared to receive quite a few of the level 5s mixed in and not so many level 100s.
Do you want a mix and completely exclude the level 5s? Advanced SG will drop 3-4 that are level 20 or 100, no level 5s, any color. What makes it trickier is the SG spawn isn't guaranteed, and the drop rate on the 100s isn't guaranteed (though it is quite high, especially compared to the dailies), so you can end up "wasting" stamina if manyus are your #1 priority (if you are making good use of the XP from 12-3/12-5/13 and don't mind the 2*s to sell and occasional ampy, then going for SGs is likely much better)
One thing you can do with them is use one or more of them right before burning 10 others if it isn't going to get the target girl to a new level. This lets you get 11, 12, or 13 manyus worth of XP at the same price each.