If I recall correctly, enemies in POE do not respawn, so running out of camping supplies simply means a tedious return to a town to get more, with no danger on the way out of the dungeon/area, nor on the way back in to where you were. Also, resting does not incur any risk of enemies attacking while you are trying to rest, unlike BG.
Yes, POE spells are not per-encounter, but you can select the best spell from a given level pool in each battle with no need to prepare it in advance. Kind of the opposite of strategy. Tactics, but not strategy.
95% of my D&D experience is with AD&D 1st Edition, where clerics certainly did have to memorize spells. I thought 2nd edition was the same, but I always played mages, not clerics, and it's been a long time.
I remember house rules some DMs would use that clerics could cast Healing spells instead of a memorized non-Healing spell, using up the memorized one in this way, but the non-Healing ones always had to be memorized. I thought there was a mod for one of the IE games that did this, but the memory is hazy. In fact I think 3rd edition used some variation of this old house rule. People used this rule because otherwise clerics always memorized healing spells, since unlike everything else, they were guaranteed to be useful.
I'm not sure what you mean by "UA," I pulled my copy of Unearthed Arcana off the shelf, but it says nothing about clerics being able to cast spells at will.