custom/tag fields idea
Oftentimes batches have information that is not standard with the guides/printings on the paper. What if indexers could have a "add custom field" option at the bottom of the entry that has a drop down of all possible field types where indexers could click on that field type, and a field of that type would appear on that entry (they could also delete it)? That way researchers could trust that the indexers gave all the information of the record (as they understood it) so the researchers don't have to learn what the indexers knew by relearning it from the record image itself, or worse yet, letting valuable information be lost in a record they can see themselves. Also, people new to a project would have a less frustrating time indexing because they wouldn't have to get used to letting go of useful information.
I think you're confusing indexing with transcription. They're not at all the same thing, and serve totally different purposes.
The purpose of an index is to help find the records. Capturing every little factoid from the page would just slow down the indexing, without making the records any easier to find. When you find an indexed record, you're supposed to use it to look at the image that it was based on. That's the whole point! Since indexers are only human, you may come to a completely different conclusion about what the record says.
Yes, sometimes all we have easy access to is the index, not the record itself, and it can be frustrating to know that more information exists. But unless it's a case of destroyed records (which are thankfully rare nowadays), there's almost always a way to track down the original -- after all, the indexer had to be able to look at it.1