Steam would be a good thing.
I have yet to hear a valid argument for how it would be detrimental to Frontier and the community for more exposure.
Most of the time, people who argue against it have a poor understanding of what Steam is and how it actually works. Or, they had an awkward experience with it that made them biased against it.
Even if on Steam, Frontier would still be in control of the game. You'd still have to use Frontiers launcher to update and start the game. VAC wouldn't be an obligation, nor the OTA file management functions that would handle game updates. Steam wouldn't even obligate you to have it running in order to play Elite Danagerous, only if you wanted the community features and overlay available while you play.
My Steam has a program short cut for Elite Dangerous already that starts the game, operates the overlay and updates my friends list so others see when I'm playing.
The question for Frontier is only a matter of if the added exposure would be beneficial to the game or if it'd simply create a burden on them to manage accounts created through the FD webportal and Steam.
We're talking about logistics and exposure. The end-user experience would remain unchanged for players.
That you can add any game to your Steam game start menu is irrelevant.
I have seen many games suffer the effects of sudden, massive exposure only to fall quite quickly once the crowd leaves.
That Steam has gained a reputation for promoting bad games, abandonware, failware, or cheap skins of other games has made me extremely selective. Unfortunately a game flaw will have a hundred advertisers for every person who says the same game is worth playing. Also, the propensity of Steam games to be derided by people who never learned to play any game with more than the number of buttons on a Playstation or Xbox controller has means most top Steam releases are console games with terrible PC ports because the developers are pretty much forced to dumb down their games for the simple game player of today who grew up never knowing anything except cheap console games that lead you by the nose and make everything easy mode.
With very few exceptions most people who aggressively promote a Steam attachment aren't the type of player ED needs or necessarily wants. ED will never be a multi-million player game. It really only needs 40-50,000 core players at any given time. That will be enough because a slow but steady trickle of new players will replace those who take extended breaks.
There are many good niche games on Steam but a lot of them are really boutique projects by miniscule development teams or art projects where the concept of a playable game is taken in completely different directions. I've been a Steam player since it first started collecting a stable of games and marketing them, and I'm sure the benign neglect that describes Steam on a good day will not do a game like ED any favours by expanding to a large audience it may not want.
Good word of mouth is much better than 100,000 Steam players who expected point-and-kill space plane flying like they would get in a console game and instead get a game that actually requires a little skill and is a little hard core.