How important is it to
you to keep your pack stowed under cover at your campsite? (Eg, in your tent or your tent's vestibule, or under a tarp). Or what other reasons do you really need a vestibule for?
Up until about 6 years ago, my only tent at the time had no vestibule at all, and I was perfectly happy to leave the pack out in all sorts of horrible weather. The pack was waterproof enough to not get wet inside, and the gear was all in waterproof bags anyhow. I also had no problem cooking with the stove just outside the tent door in the pouring rain, and opening the door occasionally to poke my arm out to stir the pot. I could just cram my boots between the tent and the fly (with gaiters over them) to prevent getting mud in the tent and water in the boots. Back then it didn't bother me in the slightest that my tent had no vestibule.
These days, I actually unpack almost the entire contents of the pack into the tent (except for the contents of the front/top pockets, which is odds and ends and waterproofs). I figure I'm going to wear/use/eat-from almost all of it anyhow, and have to empty the pack to re-pack it later, so why not have it all handy in the tent? This way, I also don't have to worry about getting wet to get at anything in particular.
And yet now that I've got huge vestibules in both of my current tents, and almost nothing in the pack when camped, I usually do make sure that the pack is stowed in the vestibule. The only time I leave it out of the vestibule now is when sharing my large tent, and wanting one of the vestibules empty and I can't be bothered trying to balance two packs on top of each other in the other vestibule.
Still, it doesn't trouble me much to leave the pack out in the rain overnight, or to cram my boots between the tent and fly, or to have the stove out in the rain, while I'm either in the tent or wearing wet weather gear while cooking. So I still don't really feel the
need for a vestibule. But I sure like the luxury of it.

It seems that most other people here like to have large vestibules in their tents, and the stated reasons are usually for stowing packs to keep them out of the rain.
So how important is it to you?