I've mentioned this story a couple of times before but I'll repeat it here to illustrate the importance of tempo:
When I was in choir, the director always focused on expressivity, getting the phrasing right, dynamics, vocal balance, etc. However, his one flaw was the tempo: he was almost always slightly slower than the tempo best needed to express the pieces. Even though we rehearsed the expressivity a lot, for some reason, we never got the music; it always felt as if we were singing without purpose. Then, one afternoon, we rehearsed a song slightly faster than usual. If I recall, the director was a bit rushed because we had to go through many pieces so we were limited in the number of run-throughs. After the third and final run-through, it felt different. We were all quiet. I looked around to see their faces and it had a calmness that was never there before. I felt different, like I was touched. On technical matters, our intonation was perfect, vocal balance was perfect, dynamics and phrasing were perfect. Musically, everything about that song felt almost etherial like god was resonating through us. It was the first time we got a piece and felt what it meant. But like all good things, this feeling didn't last. The following week, we rehearsed it at the normal (slower) tempo and it all fell apart. Intonation slipped, balance slipped, dynamic and phrasing slipped. And we were chastised for it. He blamed us but I knew it was because of the slower, less musically expressive tempo that caused these issues.
Here's another story, this time about the symphony orchestra. Now, the symphony orchestra was not a good orchestra by any standard. Intonation was a huge problem and it was very obvious even to non-musicians. The previous conductor always, like the choral director, focused on musical expression. He assumed that by going slower, the students would be able to play together. (He was severely wrong, of course, as are any other conductors who believe this myth.) As a result, just like the choir, they sounded poorly. But during the semester that the school was looking for a new conductor, one of the violin professors, an international concert violinist, temporarily took over the position. The results during the first orchestra recital was dramatic. Like night and day! They sounded like an entirely different orchestra. The musicians still had technical issues with intonation, but it was much less noticeable.
One afternoon, I decided to sit in on their rehearsal to see what he was doing differently than the previous conductor. It was immediately apparent that what he focused on was very different. He focused on feeling and expression. He had them rehearse at the musically appropriate tempo so that they could feel the music - he never went slower just so they were together. He was very animated with his own body, moving with the music, trying to show what the music felt like. I could see that the students were struggling with the tempo - they weren't used to playing that fast. But the results were immediately apparent. They sounded like musicians, not students. And during recitals, it showed.
There were gripes amongst the students. However, these gripes were more at their own inability. Since the conductor pushed them, they had to extend beyond their capabilities and this caused frustration. But the one thing they never complained about during this semester was the quality of their performances. They sounded much better than they ever did before.
But like all good things, it had to come to an end. A new conductor was hired and conducted the pieces slower than the tempo needed to best express the music. He conducted slower so that they were together, and to get the expression right. However, he did so at the expense of the music. I sat in the back of the orchestra a couple of times just to feel what it was like to be in the orchestra. Yes, it sounds very different sitting in the strings section than in the audience, and even though you can only hear your section clearly, you can still hear whether or not your section is playing musically or not. It was slow.
A couple of semesters later, a competition-winning string quartet became artists in residence. The 2nd violinist was exceptionally good (the best I've heard from even the most famous concert violinists, past or present; she's that good!) She played in the orchestra and one time I overheard her say something quietly about the tempo: it was too slow. She was obviously dissatisfied and she wasn't the only one; the other members of the string quartet didn't like the tempi, either. She eventually stopped playing for the orchestra, but I don't know if this was due to her dissatisfaction or the string quartet's touring schedule.
The point of these stories is to highlight how important tempo is in musical expression. Notice that the director of the choir and the orchestra conductors (except for the concert violinist) all believed that rehearsing slowly and with musical expression would result in the best performance. However, this very obviously wasn't the case. All those details that they spent so many hours rehearsing simply fell apart. Why? Because the conductors chose the wrong tempo. It's a myth to believe that playing slower but with great musical expression will magically cure poor musicianship. It simply won't. Tempo is even more important because expression is controlled or limited by the perception of time. If you get this part wrong, even if it's musically expressive, you undermine your entire performance. Even though expression is very important, real musicians are not fooled simply by it alone.
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