Also, a lot of people don't realize this, but if you close your SC w/ Helix, the MB window is extended by quite a bit. So Ionohelix instead of Thunder to close Fusion, etc.
The thing is, due to the penalty for repeated same element nukes, that longer MB window is often not terribly helpful anyway since your 3rd nuke in a MB window is going to be crap regardless (especially with multiple players nuking). It also makes it somewhat more likely that your mages will be used to the more typical non-Helix Immanence SCs and accidentally waste a more powerful nuke by casting slightly too early on first spell of a MB (which outweighs the benefit of tacking on some weakened 3rd nuke in a single SC window).
Not an issue if you're doing something like a SCH solo. Or maybe MBing with a single BLM who is used to the SCH and expects the longer SC window and initial SC startup time... but I normally don't find it worthwhile to bother with in most party MB settings.
Personally, when I'm on BLM it tends to irritate me when I run into a SCH who uses the longer startup, because it's harder for me to time nukes based on watching the Immanence spellbook animation (which works like a charm for the more standard non-Helix SCs).