I'm not sure I understand your title.. Are you saying is it worth buying the Dark Barracks if you're an early TH7? If thats your question, then my opinion is no. At TH7, I never collected enough Dark Elixer to train up a DE army. However, I would still get a DE Storage... Also, another thing to consider is the DE you could possibly get from Clan Wars, if you attack the right bases, you can get lots of DE, and in that situation I think it would be smart to build the Dark Barracks.
I'm still pretty early th 7 and got my bk right away. Didn't bother with dark barracks though even though I still have about 7000 just sitting there. It's pretty easy to get in crystal and gold. I figure once I build my base and troops up more I can climb back up and go after it later. It was annoying searching for town halls to snipe to stay in those higher leagues. I don't know if it was because of the update coming up but it seemed to get much harder to find town halls to snipe.
I'm debating building dark barracks now but there is just too much to update, not sure it is worth it. Guess I will just slowly watch my de that I farmed slip away.