If you actually watch Subspace Rhapsody you'll see that it is far from silly. Yes it's very lighthearted but it's actually an intimate deep dive into the characters.
That's actually usually the case for musical episodes. They have a reputation as fluff, but in fact they're often key events in the story and character arcs and the resolution of major conflicts. People tend to give
Buffy's musical episode credit for starting the trend, but
Xena: Warrior Princess's "The Bitter Suite" beat it to the punch by three years, and that was a crucial story arc episode, the climax of an intense conflict between the lead characters.
I think the trouble is that some people find a series fun when it plays by its universe's rules and some people don't care about that and just want it to be imaginative.
Those aren't conflicting goals. It's a myth that imagination is about pulling things out of thin air. It's really about taking things you know and seeing new ways to put them together and draw out their potential -- like how a chef creates new recipes based on an understanding of how existing ingredients taste, how to prepare them, and what their possibilities are. Applying imagination to an understanding of a setting's rules can produce more creative results than applying imagination without that understanding, because you have more raw material to work with, and conversely because working within constraints can inspire creative ideas to work around or within them. As a science fiction writer, I've always gotten my best ideas from understanding the rules, whether the real laws of physics or the continuity of a fictional universe like
Star Trek, and exploring the possibilities they suggest.
I thought the puppet idea was fun in Angel because it suits Angel's magical universe. I thought it was fun in Legends of Tomorrow because Legends of Tomorrow is nuts. It would not be fun if it happened in The Pitt.
Yes. It should really depend on the approach and intent of the universe. Gene Roddenberry wanted
Star Trek to be more grounded and plausible than other SFTV, scientifically literate and believable except where dramatic license or budgetary and logistical limitations required going a more fanciful route (e.g. with humanoid aliens, Earth-parallel planets, or magic instant translators). Other creators have frequently fallen short of that, and Roddenberry himself certainly did more often than he probably realized, but it was the original intent, and it was part of what set
Star Trek apart from the rest of SFTV from the '60s through the '90s (since, while it often fell short on the plausibility front, it was virtually the only show that even
tried to have any trace of scientific literacy or credibility at all). These days, we have other shows that do plausible science fiction pretty well (e.g.
The Expanse and
For All Mankind), so it bugs me that ST no longer stands out from the pack in that regard.
"Subspace Rhapsody" was a pretty good episode, but the handwave for why the characters were singing was utterly nonsensical, and I wish they'd come up with something better.