I know everyone's entitled to their opinion, but quite frankly if anyone objected to carefully cleaning dirt off a loose figure or its accessories, then as far as I'm concerned they can go forth and multiply themselves! It's perfectly fine and actually a good idea in most cases as not only does it improve the look of the figure and return it closer to its original (and intended) condition, removing the dirt (depending on what it is of course) can actually prolong the life of the soft goods. Dirt is a breeding ground for fungi and bacteria which can cause permanent damage if left to their own devices.
As for washing them, personally I do what you did and always recommend hand washing soft goods items in COLD water and a suitable gentle cleaning agent. I start small and work my way up if I don't get the desired results. Water alone can often remove light dirt, however using regular soap as well can work wonders. If that's not tough enough, I use a little non-bio washing powder and carefully and repeatedly knead and rub the soft goods to remove the dirt. The process can take a while, so don't be impatient or too rough with the item, and you should see good results.
The idea of placing the soft goods in a knotted sock and then in the washing machine doesn't sound terrible, providing the item isn't fragile and that you use a low temperature (30 degrees C). High temperatures can and do cause shrinking with many fabrics. Remember, Star Wars figure soft goods weren't designed to be washed, so many will shrink if you're not careful. Overall though, I prefer hand-washing as you have more control over the item and can stop cleaning when the item is as clean as you want it to be. Just remember to thoroughly rinse the item free from soap / detergent before leaving it to dry as flat as possible, or you may have one very wrinkled soft goods item!