I own an Oxford Stormex cover which quite durable and thick and I thought it would keep my bike in good condition. There's been several times though that I've found moisture on the bike underneath the cover and it can't escape. Is this doing more damage in the long term to the bike instead of leaving it out to endure the elements?
Before I moved house last year my bike lived outside all year round under a stormex cover, if you leave it it will end up rusty and mouldy as condensation forms on the bike and doesn't evaporate.
If you aren't using the bike over winter the best thing I found was to put it in a vac bag first then put the cover over it, you can then forget all about it until spring.
Give it a good clean first, remove the battery, wheel it into the bag and use your vacuum cleaner to suck all the air out before you seal it up. Works very well, my bike was always spotless when I uncovered it.