Disable source for metric index, any downside?

I have an index that is all metrics.
I have disabled _source and _all for entries in this index.
I have verified that I can do aggregate queries against this index.
I am assuming that kibana only cares about is the returned aggregation information and doesn't care about source.
I'm struggling to determine any upside for keeping _source around for a metric index.

Not only do I plan to visualize my data using Kibana, but I also intend to do ad-hoc queries against my data. When I disable _source, I find that I can still get ad-hoc search results by adding a fielddata_fields child to the _search endpoint.

I use a search syntax like this:
GET /access-log-lines/v1/_search
{
"query": {
"bool": {
"must": {
"prefix": {
"http.request.host": "www.example.com"
}
},
"must_not": {
"match": {
"http.request.UA.name": "Chrome"
}
}
}
},
"fielddata_fields": [
"http.request.UA.name"
]
}

I get output from that ad-hoc (non-aggregate) search query that looks like this:
...
{
"_index": "access-log-lines",
"_type": "v1",
"_id": "314159",
"_score": 1.0,
"fields": {
"http.request.host": [
"www.example.com"
],
"http.request.UA.os_name": [
"Other"
]
}
},
...

For this sort of use case the only downside would be that you cannot reindex.
This may not matter much to you, but there are some changes around strings in 5.0 that may have you second guessing disabling things.

Okay, so it sounds like the roadmap is such that I shouldn't worry about disabling source.