Obis are intensely situational for a BRD, and there's only one or two places where you can count on weather to be up all the time to even test the theory.
Limbus and Dynamis are two of those areas. I had a light obi for Limbus on my BRD, and a dark obi for dynamis on my RDM.
A further note on Limbus: Requiem appears to be generally more accurate in this zone. On things that aren't immune, it will tend to land. I wore the obi for kicks.
To answer the OP's question more precisely:
It's rare to have dependable weather. It usually isn't worth it for anyone who's not a direct magic-DD of some sort to carry Obis. Bards rarely get weather buffs from SCHs, and they're definitely not going to be light weather or earth weather buffs. Weather in general is a weird sort of phenomenon.
Should obis be included in every bard's kit? EH. No. You can bring the light one with you because Requiem can be a little variable in the resist department. Elegy is a ridiculously accurate song and there's more important things to bring with you -- like a full kit of instruments, gear, and food.
If you want to be good at HNM Elegy, have /BLM available and cap your Troubadour merits (and Nightingale, if you're feeling especially committed).
Edit: The things would be pretty useless in a merit situation ... where in all likelihood you'd be wearing a Haste belt, an MP belt, or a CHR belt. The bonus from weather effects is so small and you have so many other things going on that it'd be an ugly swap to manage. Not all weather shows up in all areas, either! I can't say I've ever noticed a tangible difference using or not using Obis, even in Limbus. The most tangible accuracy boosts I've ever acquired on BRD are, in order of magnitude: capped bard merits, HQ staves, and valkyrie's coat. Valk was just the icing on the cake.
Edited, Jun 10th 2010 10:25pm by Ezzelin