Hello there!
Do you wish you had more freedom in pairing up your characters outside story limitations? Are you a hopeless romantic who felt cheated when a minor love interest was ushered away and forgotten? Did you recreate them as a character despite this, just so you could RP with and by extension continue their story when the game abandoned them?
Ironically, the concept of turning player characters into companions is not a new idea. I first witnessed this back with the original Guild Wars, where you could adventure alongside other characters from your account and create a unique and fully customizable party. Purchasing these 'Mercenary Hero' slots allowed you to 'hire' any level 20 character on your account as a Hero, yet they weren't any different from normal companions in their effectiveness, simply adding the illusion for more individuality, (especially for RPers who want that little something more to complete certain looks.)
And who doesn't want that? With the impending arrival of allowing characters to adopt different classes there could also be a market for this? True, GW1 is a much different game yet that shouldn't be an issue when TOR already has fairly solid foundations to implement it. For instance, the Cartel market could include a 'Player Companion' slot for purchase, much in the same way you can buy additional Character slots, which are of the same cost yet once activated only unlock a slot option per character. The player is then able to meet with a NPC, much in the same way they would visit a NPC at The Cartel Bazaar, in order to obtain droid and creature companions.
During this transaction with the NPC, the player is then given a list of their character names and once one is chosen, viola! That character becomes available as a companion, who fights just like any normal companion and comes with the typical customization options found with earlier companions. Also, each character companion's combat style will revolve around their class at the time, meaning a Jedi Sentinel character will fight similarly to a Azshara Zavros companion... while a Bounty Hunter Mercenary character could fight like the Shae Vizla companion.
So, at the end of the day this wouldn't make other companions useless, just offer more options.