I was working on a coin-op game for a mid-level manufacturer in Chicago. Let's just say they were the last dinosaur in the tar pit and they didn't have a stellar hardware engineering team. They tended to copy other people's designs and not really understand the architectures, so things like in-circuit emulators and IDEs were scarce or non-existant. You debugged via trial-and-error and, if you were lucky, printf().
So I was working on this title and started working from home a couple of days a week to take care of a family member. That made things hard, and even harder since you tend to have the physical machine next to you while coding. It was frustrating to code for a day, get to work, then find half of it wasn't working.
Then I discovered that the MAME kids had already supported the platform. With a small amount of work I could develop on the emulated platform. I could work remotely and I had access to things the in-house guys didn't even have: hardware breakpoints, live RAM viewing, scripted testing for example.
Without MAME I would have seriously been suffering on this project.