IMO, you want the flexibility to create either: (a) permanent devbox VMs, and (b) per-task VMs
Agent sandbox platforms tend to be tuned for the latter, which sometimes involves VMM hackery for fast boot, snapshotting VM filesystem and RAM, etc.
Some workflows are a lot simpler if the multiple agents share a VM. These are workflows where agents must share state. A simple one we have: making related changes in our public OSS repo and our private repo, and then testing the change.
And other times you want to split up the tasks onto isolated VMs so they don't interfere with each other (ie run two dev servers without database or port collisions).
I tweeted a bit about this (https://x.com/dbmikus/status/2099264325231771878) and had a little debate with folks about ephemeral vs persistent VMs for agents