When cast on a friendly it renders them immune to slipping. Rime also provides you with a way to potentially stack many grit saves in tight corridors or chokepoints, which is particularly useful earlier in the game. If memory serves, the melted ice turns into water, which can be used to great effect in setting up lightning surface traps.

Yep, it's definitely a more niche spell than others. It also will lose much of its luster in the later stages of the game where enemies regularly have immunity to slipping.