I'm not sure if we are at slight cross-purposes here. If you're planning to use the membrane within the beds simply to line the edges, that's one thing, but if you're planning to use it as a base lining for the bed to prevent weeds coming up through it, that's a different thing.
The membrane, if used as a bottom lining, will also stop your plant roots getting down into the soil. I think that plants such as potatoes, many carrots, parsnips, sweetcorn, tall brassicas, will all need a depth of more than 6". So it depends on what you want to grow in there. You'd be fine with more shallow rooted plants that are unlikely to suffer with wind rock.
Given that you've bought these now, I'd use them - I don't see a problem with them for the short term as long as you're clever about what you grow where.
Definitely use the membrane to cover up the bits you've not got to yet. I resorted to that one year when I knew I wouldn't be able to spend much time on the plot. Better to have a plan to stay on top of the weeds, than let them get on top of you! You're doing fine