I used to bivvy quite a bit, initially with an orange plastic bag, and later with a simple goretex bag. I didn't worry about midges or slugs, and mostly slept with the top unzipped and my head out, just zipping up if it started to rain. I didn't have any problem with leakage, even after finding my feet in the water when the river decided to overflow into the A65 pedestrian underpass at Clapham. These days I use an Akto.
One thing that you will need for more than occasional use is some form of additional shelter, such as a small tarp. If you suss out a bivvy spot, go to the pub, then find it's chucking it down when you come out to sleep, what do you do? Getting your kit unpacked and you out of your clothes and into a sleeping bag inside a bivvy bag without getting your kit pretty wet isn't terribly practical when the only shelter is the waterproof jacket you are wearing. The reverse, waking up to torrential rain, is not as bad but still difficult if you've just hopped over a drystone wall or something.
An alternative to a tent or bivvy, for lowland use at least, is a
Hennessey Hammock. I sleep front/side rather than back, so I've not tried one, thought you are meant to be able to lie flat.
With a tent like a Laser Comp coming in at the 1kg mark, there aren't really any weight advantages whatever you use. It's more a case of available and discreet sleeping spots.
PS
I believe that Teethgrinder knows all about being bitten by slugs