Developers Delight was probably intended to aid development of RFTools and maybe Deep Resonance, but I personally found it to be very useful.
Even though I'm not developing any MC mods at the moment, this would have been very useful fixing the effects of a bug in Deep Resonance we encountered. (@awilliamson is currently trying to fix it and will make a PR)

Since it's an admin only tool anyway - Unless there are technical reasons preventing this, I suggest allowing the tool to modify NBT data.

👍 +1

Developers Delight, I believe, would benefit greatly from a more interactive utility rather than pure introspection. This would have certainly come in helpful during debugging rather than scouring through the entire NBTExplorer for an entity to modify it, then replacing specific .mca files within the server. Very manual, and I believe it should be made to be a more fluid process.

As @Xandaros mentions, it is admin-only, and therefore should be fine from a user-level.