Hi Thistle,
Its not silly at all. If it keeps things from getting messy etc then its all good.

I had the messy problem when having to apply cream practically all over my legs and on patches on my arms. This obviously caused anything that I came into contact with eg, bedsheets to get all sticky and messy.
Well because the patches are so big on my legs etc I found that after applying the cream to the affected areas the best way to keep things from getting messy was a method similar to the one you have mentioned.
I found the best way to cover my badly affected legs was to put a hole in the middle of a black bin liner and then wrap it around each leg and put a little bit of sellotape on it to keep it secure and then it was ideal cause I could just rip it off in the morning and then get showered and ready for work. Its not ideal for all over the body as it could get quite sweaty, but for the legs or feet, I think its ideal.
What do you think, its a similar method to you using clingflim?