Is it worth spending orbs on Summer Tana?

Heck yea! Basically a better blade tome Hnowi.

If you have HNowi i would rather use her. Yeah Tana is better option but Nowi has hone flyers wich is irreplaceable if you have another mage flyer or fodder her off for that skill. Still the main source of her power isn't stats alone but blade tome LnD and buffs.
But that does not apply if flyers are your core arena team.

She's the second best tome flier right now imo (after SF!Nino). I got one +Atk -Res the first day of the banner and I love using her right now.

She’ll be around for 32 days, so i’d wait for a bit to see what else is coming up. I’ve borrowed one in RD and she was pretty fun to use, but a leg banner is coming up soon, I think.... so i’d wait to see what’s on that one. :)