The speaker functions like a judge evaluating two arguments. At the beginning of the poem, we do not yet know whether he will weigh in on the debate.
Some say in ice. (line 2)
Oh, it's on. "Fire" and "ice" are pretty arbitrary phenomena, as far as ways to end the world. Why only these two? What about being sucked into a giant black hole? Giant Godzilla attack? (OK, so you can probably file that under "fire.") The point is that Frost doesn't care about being strictly accurate; he is setting up two contrasting symbols.
I hold with (line 4)
The speaker uses a very legalistic phrase to let us know that he's planning to take sides in the argument. He sounds like he's competing at a high school debate tournament.
Is also great And would suffice. (lines 8-9)
Wait, so what was the choice? The speaker kind of skirted the issue. The end of the poem suggests that it doesn't matter whether fire or ice ends the earth. Nobody is going to be around to say, "I told you so!" The speaker wants to show that ice is an underrated force of destruction.