MLT Component doesn't let people highlight/paginate and the handler comes with an cost of maintaining another piece in the config. Also, any changes to the default (number of results to be fetched etc.) /select handler need to be copied/synced with this handler too.
Having an MLT QParser would let users get back docs based on a query for them to paginate, highlight etc. It would also give them the flexibility to use this anywhere i.e. q,fq,bq etc.
A bit of history about MLT (thanks to Hoss)
MLT Handler pre-dates the existence of QParsers and was meant to take an arbitrary query as input, find docs that match that
query, club them together to find interesting terms, and then use those
terms as if they were my main query to generate a main result set.
This result would then be used as the set to facet, highlight etc.
The flow: Query -> DocList(m) -> Bag (terms) -> Query -> DocList(y)
The MLT component on the other hand solved a very different purpose of augmenting the main result set. It is used to get similar docs for each of the doc in the main result set.
DocSet(n) -> n * Bag (terms) -> n * (Query) -> n * DocList(m)
The new approach:
All of this can be done better and cleaner (and makes more sense too) using an MLT QParser.
An important thing to handle here is the case where the user doesn't have TermVectors, in which case, it does what happens right now i.e. parsing stored fields.
Also, in case the user doesn't have a field (to be used for MLT) indexed, the field would need to be a TextField with an index analyzer defined. This analyzer will then be used to extract terms for MLT.
In case of SolrCloud mode, '/get-termvectors' can be used after looking at the schema (if TermVectors are enabled for the field). If not, a /get call can be used to fetch the field and parse it.