Let's think about this for a minute. Its a nearly impossible task to find this droid and recover it. (in-lore) and very hard and annoying to get him in-game. He's a killer who thinks anyone organic is a target expect the lucky person who controls him (it's master).
Why can't you use him in lower levels? Why because your character is too weak to, at 50 your character who destroy the forces that were guarding the HK unit is more than a match to control this crazy droid. I can't imagine a level 25, 30 or 35 character looking for this droid and surviving, plus for those people who are still leveling, getting the HK is additional content. I believe you can unlock him in your legacy where the quest is a lot easier to do with another character. Again for a level 50, yes this isn't much content yet, but when I roll my smuggler I want him to earn this droid, so by the time he is at 50 and has gone through all this crap and lived, he wants to find himself a powerful HK unit to watch his back.
When it comes to content, yes the pre-50 has decent content, I only remember a handful of moments where I enjoyed the planet's storyline and my own class storyline but end-game content like Ilum, black hole daily's, and the Belsavis daily's give a great opportunity to wreck chaos with HK-51. Plus I want to earn this bad boy, I don't it handed to every one of my toons at a early level, that doesn't work with your other companions, why would it work this time? Bioware has a lot on their plate and they're trying to make us happy, take it for what it is and try to enjoy, with a view like your you'll find faults within everything and ruin the entertainment value of it before you had the chance to enjoy it.