Roost still allows Ho-Oh to setup on Ferrothorn, Forretress, Grass Arceus ( repeated Ice Beams can slowly wear out Ho-Oh ) and Blissey. Ho-Oh easily scares them out, so it gets plenty of opportunities to Roost back. If Stealth Rock is up, Roost is even more useful and you don't have to switch out to recover.
0 SpA Ghost Arceus Judgment vs 0 HP EV Ho-Oh 29.2% - 34.8% You can easily survive that and Roost off the damage while Sacred Fire is an easy 2HKO in the sun.
When using Tailwind, I think Ho-Oh should use Adamant nature for maximum power. Most Excadrill run Adamant anyway so I think Adamant should be main slash.
QC APPROVED 1/3