My ranger is an auto, but another vehicle I own is a manual. There are times when it's a hassle to push the clutch down to start the engine. There have been times when I've been working on it, gotten nasty filthy greasy, needed to turn the engine over, and then have to be extra careful about not messing the interior when I go to crank the engine.
Like I said, sometimes it can be a hassle, but I leave the interlock there. I don't know if I've ever accidentally tried to start it in gear, but I probably would have at some point if the interlock wasn't there.
It's also one of those theft deterrent side effects of having a manual tranny vehicle. I've seen a number of news stories where the criminal failed to steal a vehicle because they couldn't operate a manual tranny. The interlock is just one more thing they have to master.
but its his truck, I've seen lots of things people do to their vehicles I wouldn't do.
Hook it up to a switch, so you can disable the interlock when you need it disabled, and otherwise leave it enabled for those days when you're in a hurry and might not be paying full attention to all the details.