It may not be a "big" part of your damage, but it's still going to be a decent chunk. Even for BLMs, the Thunder dot alone is around 10%, if I'm remembering my parses correctly.
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when did 8 man "raids" become such serious business?
I main summoner, but I leveled BLM to 50 as an alternate, and ironically, half the time I run as BLM (especially in four man dungeons), my FC mates lament not having my summoner for a back up raiser. BLM also doesn't have quite the mobility that summoner does. When I asked my FC if I should level something besides summoner, they said "Only if you really want to." So I think if you are in a Free Company willing to kick you out after this patch, you are just in a group of elitists and are probably better off without them anyway.