I inherited an environment that had no cable management arms, and we've slowly been managing to get them installed.
The reasons the previous admin used for not purchasing/using them were cited above: You would not be unracking a live server, they interfere with airflow, and you should be trying to reduce the amount of cable in the rack, not increase it to deal with the full span.
The problem shows up when you're maintaining a heterogeneous server room over a number of years instead of installing an entire racks of servers at once. We have three manufacturers of servers and usually 2-3 generations of each in production. We add or remove machines every three months.
- We have equipment arriving and leaving constantly.
- We don't have the opportunity to zip-tie things to lacing bars -- we don't have enough space to give up a 1-2 U to them, and we don't want to "layer" things because we'll always end up digging the oldest cables out of the bottom layer.
- We don't get to pick or focus on one vendor because I work for a university that receives grants (sometimes from hardware manufacturers) and relies on a public bidding process for large purchases.
- We have three to four Cat5 spans to each server -- typically one for internal network, one for public network, one for KVM, and one for the ILO management port.
- Some servers are also attached to fiber (and we run the fiber inside a small conduit to keep it from pinching), while others have an additional 2-4 cat5 cables running to teamed network interfaces.
- Then there's two power supplies for each server.
I'd like to see anyone make a clean server cabinet with that many cables running to each server unless they use cable management arms of some sort.
In a "rush" environment, we're able to pull a server out without walking around to the back first. We know what cables are being plugged or unplugged because of their colors.
There's many reasons not to use cable management arms, but when you're working in a typical business environment and not a engineered environment, they're really worth it.