Hi
A pack cover is largely redundant if you have a poncho with sleeves like the
Altus (I have a Raidlight which is the same basic design). However, do note that if you need to take the pack off for an extended period when it is raining, say to pitch a tent, it will get wet.
For extra rain protection, many people use a pack liner such as a garbage bag, which weighs next to nothing and can double as a carry bag at your destination. Also, some like to use waterproof "packing cells" (Ziploc bags are cheap and light weight) which aside from extra rain protection, makes packing and unpacking simpler.
Yes, trying to don the
Altus especially in windy conditions is difficult IF it's put on like a jacket, one arm at a time, as the pack gets in the way. However, there is a very simple, sure-fire way that works every time for me - I store it folded with the zip mostly done up and when I need it, I just pull it over my head like putting on a sweater. I tend to scrunch it up from the bottom and put my arms in the sleeves first; once it's over my head, it's quite easy to pull it down over the pack. This technique works even if your pack sticks out a bit, but best practice first in front of a mirror. By the way, if it's blowing a gale, face INTO the wind otherwise the coat may invert like a blown umbrella!
Like most of this design, my Raidlight is quite voluminous so I can pull my arms out of my sleeves under the coat to access the pack waistbelt pockets and drink bottle pockets without unzipping the coat.
Hope these tips help!