My opinion* is you should grow as high as you can reach without the runners affecting other plants. If they want grow higher, trim them. Above all, don't do anything that would cause you to miss picking ripe pods!
The point of growing beans is to harvest as many pods as possible, with the possible secondary purpose of putting nitrogen back into the soil as part of a rotation (but that's not relevant to plant height). Thus the principal downsides to allowing the beans to grow too high are:
- You can’t reach the beans (especially if you're petite) and so you don't pick them promptly causing the plant to decide it's fulfilled its biological purpose and shutdown.
- Energy is unnecessarily expended in growing more infrastructure (runners) rather than flowers that would become beans. < EDIT: This is wrong. See comment.
- If you did, instead, trim the plant at the top, it would make more lateral branches and so fill in usable space at the top of the trellis where reachable beans would grow.
The downsides to allowing your beans to grow on to another plant when you already have separate trellising:
- The beans are more difficult to pick, increasing the risk of missing a ripe pod, causing the same problem of the plant deciding to shut down.
- The runners could damage the other plant when they entwine around it.
- Transfer of plant diseases could occur.
- The finished beans are harder to clean up at the end of the season when tangled into something that's still productive.
[*] Posting on behalf of a friend, Francoise, to whom I posed this question (and who's not ready to join G&L stack exchange)