My answer would be... no, there is no need to carry the foxtail if you are lvl 220+ and you got the rainbow one.
The only respawns that the foxtail would be more useful than the rainbow, as you said, are the ones which creatures have ice damage. But the ones that comes to my mind, like ice books, bursters spectre, and others useful for lvls 200+, have better amulets to be used.
Sell it girl! xD