Sorry for the late repy I am writting a warstory.
Ice beam does have nice coverage but in regards to Celebi and Breloom (and Zapdso) although you are htiing them for SE damage Milotic really wants to stay clear of them and thus in these situtations Ice Beam is best used on the switch.
Its still an excellent move to have on Milotic though, just remember that you actually have to survive an attack to use it on certain pokemon.