Undrave
Legend
If nothing else, skywrite can replace banners and help soldiers find their side in the fog of war. Banners were important because, when everybody is bloody and muddy, it can be hard to tell who's side you should be on.To be fair, it really depends on the campaign.
If you're playing in a war story, then skywrite is one of the most useful spells - you can write a message in code to your allies far away, giving your side a strategic advantage thanks to the intelligence you've given. For example "The wizard is not among them." would be a sign to proceed with a march of troops with decreased risk of being blasted to oblivion.
of course, you could mess up with people by casting the same words...
Anything that can trigger off of spoken words can be triggered by having someone ask out loud "Wait, what does X mean?" or "X? What does that mean?"Glyph of warding FTW!