Your mileage is going to vary when it comes to subplots. I feel like authors who can handle large casts of characters, or who write long-running series, or both, have a greater license with subplot. They can weave them through several stories, and slide them in more carefully.
For me as a writer, subplot is a balancing act. I have to be careful not to let side details overwhelm the story.
First, let me be clear: I love subplots. There is not a single thing that I don’t love about the stories within the story — mostly because life is a series of subplots, of character back stories and minor quests.
As a reader, I love to know what else the main character has going on. Yes, sure, this romance is nice and all — but tell me more about what’s going on about her father’s assassination. Author, you’ve been dropping hints for chapters now, and damn it, I need to know!
Also, I love the catharsis at the end of the huge story reveal. When the main plot and most of the subplots get wrapped up in a nice little bow. When done right, there’s this exhale, and it’s all clear. The whole story has focus: the threads are wrapped and the stragglers aren’t a big deal, and it’s amazing.
(In a really good book, there’s also the element of, “I totally didn’t see that coming!”)
But subplots don’t always work. Sometimes they feel tacked in for “depth,” like the hypotenuse of the love triangle that was never really a question. Other times they distract from the plot, either because they ended up more interesting than the plot (at which point: whoops, that draft needs more time to stew) or because the author preferred the subplot.
I’m guilty of both, but I’m really guilty of the latter. I have the bad habit of falling in love with characters and wanting to share all those interesting things about them, even when they’re not relevant.
My example: the novel I wrote for NaNoWrimo in 2010.
I introduced a brother (Benjamin) about 10K into the story. He was completely arbitrary — I kept mis-remembering the name of a character (Eric), and I concluded the best option was to just throw in a long-lost brother subplot. It motivated my minor characters villainy and explained his singular focus on the main character (Alfred). I really loved the idea. I wrote on it a lot.
Except that it was really clunky at the climax of my story. The climax somehow became all about the Long Lost Brother Reveal, while characters were in handcuffs and government officials were getting their hands dirty.
There was no place for it, and it pulled all the focus from actual plot of the story. So the whole thing got tossed — probably about 3K to 5K of subplot.
Once I had removed that subplot from the story, removed all the emotional energy Alfred threw in to be angry at Benjamin, I realized that my climax was weak. There was, in fact, no climax. Nothing went wrong in the great big gambit at the end. There was nothing unexpected. There was no drama. I had relied so heavily on the big moment of my subplot reveal, that I had actually neglected my plot.
I didn’t change that Benjamin is Alfred & Eric’s brother. That’s still true to the canon of the story, and I left some hints that allude to it. As a subplot, it could have actually wrecked the novel. As a background detail, it neither adds nor detracts from the value of the story.
For me, it’s a re-read bonus. (And potential in-universe story fodder for later.) For a reader, it might be nothing at all.