It's sometimes but not always possible, depending on what you're trying to do. It's part of how the plugins are structured. Like with the Grunt perk, Honest Hearts is adding that perk and the corresponding formlist that Grunt looks at; they can put into the formlist any weapon in the base game or Honest Hearts. But when they make GRA, they can't add any of those weapons to the formlist because that formlist doesn't exist within the GRA plugin - so in order for the devs to put GRA's items in a formlist added by Honest Hearts, they'd have to make GRA dependent on Honest Hearts.
Basically, any data or forms added by one plugin can't be referenced by another plugin without that plugin becoming dependent on the other (ie it will crash if it's master plugin isn't there). This isn't a problem if the data is in the master file (Skyrim.esm, Fallout4.esm, etc) since that's the entire base game and virtually every plugin is dependent on it.
There are workarounds with some things, and some things don't always need to be dependent on each other; like with the Radiant Story system they introduced with Skyrim, you can add new AI packages and dialog to NPCs that don't necessarily exist yet. For instance, Hearthfire added mechanics to let you recruit certain followers (based on certain parameters) as stewards of your home; and any DLC NPCs that met those parameters (do you have a voice type that incudes this dialog? are you a potential companion?) like Celann or Talvas Fathryon could be hired as stewards of your Hearthfire homestead. So we might see changes like this happening, just as a workflow thing.
In the very least I hope there will be an actual error message when you get a missing master error, instead of an unceremonious CTD.