For us, it's a convenience as opposed to a necessity. Could we just do all the laundry when we get home? Sure. We have plenty of clean clothes each trip to make it to the end. However, we find ourselves doing small touch up loads all the time just because we can and have actually found that at sites without full hookups, we miss being able to run it.
All that said, if you are looking at the Splendide combo units, here's a tip. The vented requires the obvious vent to be cut into the trailer. However, taking that extra step allows for larger loads, faster wash/dry time, and a much less wrinkled end result. The Splendide ventless that we have works great but by the nature of how it drys, it leaves the clothes very wrinkled upon completion of the dry cycle. Not the end of the world if you're just doing a load of unmentionables but if you plan to throw a button down shirt and your favorite jeans through, get ready for some serious ironing time when it's done doing it's thing....
Another tip is on detergent. When it says use a maximum of 1 tablespoon per load, listen... I guessed at the right amount figuring a little extra couldn't hurt and it threw the washer all out of sorts. It played hell with it's 8 million sensors and was filling with water on the dry cycle. I even brought it an hour away from home to the nearest warranty repair guy who found nothing wrong. Brought it back, put it back in the RV, and finally figured out it was because I used too much detergent. Using a 1 TBSP measuring spoon from the kitchen has cured it and it now works as advertised. I would have never guessed it could be that sensitive but based on what the repair guy told me, the Splendide's have nothing in common with a household washer dryer and any number of things being only slightly off will wreak havoc with their litany of sensors.