Most people like Fates stories and characters? Hahaha.
Characters, definitely. Fates' cast is consistently popular in the scheme of the larger series, and even people who don't care for the game's story typically find some of its cast endearing.
or or OR, maybe they were just hoping a crossover title would span more than the most recent releases.
The initial disappointment is somewhat justified. Whining about it for months on end every time the game is discussed tends to make people less sympathetic to that position.
It has one!
Celica is playable, but the History Mode map you have to complete to unlock her is based off the final map in Gaiden. Which is also the same map as in Echoes, only it's the 8-bit version in Warriors.
But other than that she's basically a guest star.
Actually, in that sense, playing Warriors first might enhance the experience of going through Echoes later - that way they can
get the significantly less impressive Warriors arrangement of Twilight of the Gods out of the way first instead of being disappointed by it after Echoes'.