I think it depends on the situation. Dual batteries are not a necessity, but if you do have a lot of auxiliary loads it is very convenient.
I ran a single large SLA battery for maybe 4-5 years to power my fridge, and technically it works, kinda. I could run my fridge and start my car. But looking at the specs, my 80ah starting battery is really only 40 usable ah, and even at that level I was still killing batteries. It's easy to burn through that much power in a day or two with a fridge in a hot environment. I have a watt-hour meter connected to the battery to monitor how much energy the fridge is consuming, this past weekend with 2 fridges connected to my battery, over a 1 1/2 day period I used over 1000Wh, so on average about 500Wh or 40Ah per fridge.
Even with moderate use, I was replacing my battery every 18 months to 2 years, and thought that's just the price to pay for using a fridge. During that time, the capacity of the battery would slowly degrade, until it really wasn't useful as a power source. It also required a lot effort to maintain because I'd have to constantly charge my battery with an external charger because normal driving wasn't enough to keep the battery topped off.
I now have a 100ah LiFeP04 that I picked up on sale for about $400. When you look at the numbers, a full 100ah capacity, half the weight, 10X the lifespan, it's a pretty easy decision. Even if I had paid full price, I think I would've come out ahead financially if I'd gone that route from the very beginning. It was a bit of a pain to setup in the beginning, but it's a one time hassle then you never have to worry about it again, as opposed to the constant maintenance and worrying from a single battery.
The way I have mine setup now, the battery isn't really installed, just strapped down in the back. This was supposed to be a temporary setup until I could figure out a more permanent solution, but I think I like it better this way. Having the battery be semi-portable has a lot of advantages. I think the abundance of the lithium power stations has basically eliminated the need for a permanently installed dual battery system.