I can search volumes that Spotlight is disabled on, with different results from what they'd be when it's enabled since there's no content index. I've seen this error on a Spotlight-disabled volume:
Code:
% sudo mdutil -s /Volumes/Backup
/Volumes/Backup:
Error, no index found for volume.
In my case, running Verify Disk with Disk Utility on that volume was enough to make it searchable again (with indexing off).
A potentially significant issue (IMO) with reindexing volumes is that certain metadata from original indexes will likely be lost. One example I'm aware of is the kMDItemWhereFroms attribute for files downloaded with Safari (and other apps?). Right now it's hard to know what the implications of this might be in different situations. Hopefully there will eventually be a "solution" for it. If nothing more, I'd like it to be clearer which metadata may be lost when reindexing so dependencies on it can be avoided.