My main purpose of having the RainFly on has been to free the bag from dirt and mud when laying in on the ground to access equipment or just peace in mind just moving it around (especially when hiking in the bush), more so than for the rain.
It fits the Prvke31 nicely with a bit of room enough to cover the tripod and other small attachments.
I rarely secure the little loops to the shoulder straps as tightening of the bigger elastic loop and the buckle strap around the backpack do a good job already - and you are more likely to need access to the top than the bottom of the backpack, which would require undoing of the little loops on top.
I have been able to access the camera easily thru the side access of the Prvke31 with the Rainfly on- just peel that little bit aside and the whole thing remains in place - I found this a bonus as the Rainfly then keeps the side access closed (keep any little things from falling out) even if you do not zip it shut straightaway when you have the camera on hand.
Other times I would have just the camera cube (without the backpack), as use the Rainfly as a sheet between the equipment and any loose objects or mud on the ground.