I am fully insured for my services, moving vehicles and for civil liability...working from home and mobile. However, I prefer working from home where everything is set up.
While Toyotaguy and Willy make a good point about the client taking some of your time, I have this as a counter "argument." Detailers are not hard to find most of the time, and one thing I've learned in sales, is that people will always choose the guy they feel they get the most
value from. Using this ideology, I encourage my clients to stay and watch me work and ask questions. This allows them to get a full understanding of what it is I'm doing, and they get to learn at the same time. Giving the client "permission" to see what you're doing shows transparency and puts them in a comfortable zone where they trust you. and you know what? I've only had a handful of people actually stay and watch/ask questions. The rest simply leave their cars, but the fact that they know they could've stayed is a big selling point. Unless you're running a big volume shop where every minute counts because you could squeeze in another car, then I think it doesn't really matter if having the client there takes you an extra hour or 2. It makes for a better experience for them and almost guaranties they will be back.
Also, I have what could be referred to as a townhouse. It's essentially a 2-floor attached apartment, but it has all the features a house has; basement, yard, garage, driveway etc. I work from my garage and have my basement set up as a "chill space" for me or my friends, but also so my clients who want to wait have something to do. Pool table, darts, foosball table (coming soon), Wi-Fi, musical instruments, magazines, TV (coming soon) and product display.
So basically, it really comes down to what it is you WANT to offer your clients OP. You can choose to do something like me where they stay and have something to pass the time, or you can choose not to let them stay like others do and save on the time you'll be working on their cars. In the end, it's really a question of what you're comfortable offering and how much you want to "invest" in what you have to offer.