"This prevents opportunistic theft but if this space is shared with others (apartment blocks and offices) it in fact serves to increase the economies of scale for prepared thieves who break into your storage area late at night with a van. It's not uncommon for office bicycle stores (with many fancy, expensive bikes inside) to be emptied out completely overnight by professional thieves."
At the shop I used to work at, we had some guys back a pickup truck through the front window of the store, load the pickup with five or so $8-10K bikes, and then drive off. We got it all on security camera and it took less than a minute. They made off with more value than most small banks keep in cash.