There are lots of 'tools' available to someone learning to become a better jazz player.
- Recordings of great players
- Method books
- A pitch tuner
- Harmonic knowledge
- Band in a Box
- A metronome
- A keyboard (for non-piano players)
- A recording device
- Jam sessions
- A teacher
Are they good or bad? It depends on the context.
Is a wrench bad? No, but if you try to pound a nail into a board with one, you'll likely hurt the wrench and bend the nail. Don't blame the tool. Blame your poor choice of using a wrench instead of a hammer.
The same is true with the tools listed above. Each has both its proper and improper use. The trick is to know the difference and maximize the benefit of the tool using it for what it's good for.
Consider a scale. Scales are an important component of harmony. Build your ear for keys and modes by running scales. Practice them slow and practice them fast in order to strengthen your articulation and range. Sing them and consider the intervals that comprise the different scales. Listen to how they sound.
But when you improvise, don't use the running of scales to mark time and imitate the texture of improvisation. Don't use them to play safe and artificial within the chordal harmony. That's the wrench pounding a nail.
Consider transcribing. Write down the notes that the masters played. Learn to play them like the recording with dynamics, inflections, articulations, and time. Study the note choices. Understand the form of the solo. Intersperse your own improvisation within their solo every four, eight, 16, or more bars.
But don't play them as your own in a performance and don't transcribe and thus imitate too many solos from your favorite player and risk your identity being replaced by his sound. As Shiela Jordan says, That's stealing someone else's voice. That's using a screwdriver instead of a brush to paint a wall. It's using the tool of transcribing poorly.
Consider a metronome. Use it as a click track in a recording where you need to play in perfect coordinated tempo. Use it as an accountability tool keeping you from slowing down when practicing the mechanics of scales, lip slurs, and interval exercises. Use it to measure and track the strength and speed of your tongue like a runner using a stopwatch to measure lap speed. Yesterday, you were 16th notes at 110. Can you do 112 today?
But don't use a metronome to ingrain into yourself a sense of click-accurate time. The pulse of jazz is not a clock. It is a human being expressing their special view of the world. It's a groove. It's the organic motion of your life. It's a heartbeat. Jazz time is neither perfect nor mechanical. Using a metronome to practice the ideal of perfect time is like using a belt sander to cut wood. It's the wrong and destructive use of a perfectly good tool.
Getting better at jazz or any form of music requires that you know how best to use the tools at your disposal. Gather the tools and use them for their best purpose and then reap the rewards.
Use these tools for something they're not made for and you'll hinder your musical development and wonder why you're not getting better. And if you misuse the tool, don't blame the tool. Blame your poor choice and use of it.