alexwolf
lurks in the shadows
So what if you are walled by Rotom-W (if by walled you mean the 3hko that you get against most Rotom-W with Ice Beam)? You wall him too, and guess who is gonna run out of life/pps first? Rotom-W is nothing more than set-up bait for Kyurem, so being walled by him doesn't matter one bit.If you're using Sub-Roost, you're probably better off using Dragon Pulse and Earth Power than Ice Beam and Earth Power. You aren't walled by Rotom-W, you get a much better hit on waters. The only thing you really lose out on is Amoongus, which isn't on Rain Teams to begin with and can't break the substitute anyway.
What other waters you get walled by, really? Let's see... We have Starmie, which is 2hkoed by Earth Power, and can't do anything back, Tentacruel, which is 2hkoed by Earth Power and can't do anything back, Gyarados, which is hit slightly harder with Ice Beam than with Dragon Pulse (5 BP more) and we have Jellicent, which is one point for you (but it also gets walled by Jellicent). There is also Keldeo, where D-Pulse comes in very handy. Finally there is also Volcarona, that D-Pulse is useful against. And only offensive variants without Roost, as anything else sets-up on you no matter what STAB move you use.
Let's see now the targets that Ice Beam is better against. First of all Ice Beam ohkoes offensive Dragonite even with MS active. It also 3hkoes Ferrothorn, and has a posibility to 2hko with spikes, while with D-Pulse and EP, Kyurem is Spikes fodder. Oh and Skarmory too, gets 2hkoed by Ice Beam, while without it, you are again, Spikes fodder. It OHKOes Tornadus-T and Thundurus-T without any previous damage, while D-Pulse never ohkoes without SR, almost never ohkos Tornadus-T after SR and ohkoes Thundurus-T after SR, ~50% of the time. We also have any kind of Gliscor, Landorus or Landorus-T, that are ohkoed by Ice Beam, while not by D-Pulse. This may be useful if your Kyurem is at full health, and you want him to tank a hit and ohko back. Then there is also Celebi, which can wall you and force you out with Perish Song if it is specially defensive, if you have D-Pulse, but Ice Beam easily 2hkoes.
So Dragon Pulse makes facing Jellicent and Keldeo easier, and helps a bit vs Volcarona, while Ice Beam prevents you from being Spikes fodder against the best 2 Spikers in OU, gets some nice ohkoes on things that D-Pulse doesn't (flying-ground pokes, Dnite, Tornadus-T, Thundurus-T) and beats any Celebi you will face.
So, imo, Ice Beam + Earth Power is easily better than D-Pulse + EP. This doesn't mean that it is always better, as some teams may prefer the benefits that D-Pulse brings, but overall Ice Beam pairs better with EP. But if you chose to use FB, then DP is they way to go, as Dragon + Fighting is perfect coverage, while also hitting many steels for super effective damage and having higher BP than EP, and preventing you from being Spikes fodder.
So to sum it up, Ice Beam + Earth Power > Dragon Pulse + Earth Power and Dragon Pulse + Focus Blast > Ice Beam + Focus Blast, IN GENERAL.