In some situations, one already knows the schema of their content, so having to declare a schema in Solr becomes cumbersome in some situations. For instance, if you have all your content in JSON (or can easily generate it) or other typed serializations, then you already have a schema defined. It would be nice if we could have support for dynamic fields that used whatever name was passed in, but then picked the appropriate FieldType for that field based on the value of the content. So, for instance, if the input is a number, it would select the appropriate numeric type. If it is a plain text string, it would pick the appropriate text field (you could even add in language detection here). If it is comma separated, it would treat them as keywords, etc. Also, we could likely send in a hint as to the type too.
With this approach, you of course have a "first in wins" situation, but assuming you have this schema defined elsewhere, it is likely fine.
Supporting such cases would allow us to be schemaless when appropriate, while offering the benefits of schemas when appropriate. Naturally, one could mix and match these too.