Are you talking about songs with singing and riffs/chords, or instrumental compositions with multiple solo sections and a different level of complexity?
A lot of this has to do with how you see your creative process and how your muse drives you.
For me, I was really into fast solos and everything, but I discovered along the way that I was more tuned into songwriting, groove, tone, and feel, and that sometimes the solos would go straight over my head (but I would be locked into the song format and fills/changes, buiilds/arrangements, etc.). That’s what I was talking about in another post where I referred to making a “pivot” or two in development. And if you’re self-identifying at the “intermediate” level, you’re leading with the idea that your foundations are a bit flexible and there is room to rebuild as you explore.
If they really are talking “repertoire,” and you want to take them at their word, I would think that would have more to do with building repertoire, i.e.: actual number of songs that you can play through to a performance level in whatever format (jam with rhythm section, solo acoustic, singing/not singing). If I was trying to learn a solo, I would never feel obliged to play intro/verse 1/chorus/reintro/verse 2/chrous 2 just to get to the solo.
But hey, if you want to play “Back In Black” 5 times through to feel the groove, I’m all for it. There is a point of diminishing returns in your practice though – if all you play is stuff in your “comfort zone,” you will definitely end up in a rut. If excess playthroughs do develop something, I would say “endurance” and “focus” is more like it. Those are important. I guess you have to be used to making it through a whole song. I’m a product of school orchestra programs, so I can’t really theorize about that. We didn’t have a choice about making it through a whole piece.
If you do want to focus on something with that, recording yourself with the track and listening critically to the playback can reveal a lot about how your playing is translating to the outside world. It doesn’t have to be complicated – even just hearing back on the iPhone or something will tell you a lot.
As far as learning songs, it really becomes a lot easier after you hit 100 or 200 or so. And that’s not that huge of a number, by the way – you might be able to play back thousands in your head right now, if you’re an avid listener. So what happens is, you start to recognize the chord movements and how the different intervals “sit” against a given chord movement – so, “here comes the V chord…aaaaaand, we’re back at I” – you know, that’s the most simple format of that concept, I guess. As you refine it, it might start to click, and then learning a song that’s not overly complex gets to be a VERY straightforward process. I’m kind of used to the session thing, so charting out a song on the spot is really natural to me. This is where something like the Nashville numbering system can really accelerate your development, in terms of instinctively seeing the inner workings of a song and how the chords relate to each other, and building your own inner “feel” for why a given chord change “works.” And you start to notice things that are vital in the writing/arranging world that don’t show up in the basic “guitar learning” thing – like, when the next change hits, do we “push” it (hit it on the “and” of 4 right BEFORE the bar) or hit it on the bar? No one’s going to teach you what’s “right” and what’s “wrong” there, but I can assure you that whether to push a change or play it straight is one of the biggest production choices that a given moment in a song can have. And, in my opinion, the first step to developing an instinct for that is by developing an awareness for it at such a level that, you know, you’re passively listening (maybe hearing a song in the grocery store), and you pick up on it. Yes, I get distracted by snare drum sounds while I’m picking out fruit in the Produce aisle. Why do you think they call it the “ProDUCE” aisle?
This sort of thing could be absolutely uninteresting to you, or you could see writing/arranging as “where it’s at” or whatever. That’s what I’m saying – we don’t know, and “it takes all kinds,” or however you want to look at it.