As someone that plays water I agree entirely, It doesn't make sense that in deserts you are presented with the ability to pretty much be just as affective (apart from mobility) as you would be next to a river or the ocean.
Also for my idea the actual block type doesn't matter, just the number of adjacent blocks. A 3-tall cactus would provide three times as much as a 1 block sapling, meaning that torrent from a cactus would be possible, but just more restricted and more difficult to prepare quickly in a desert. Meaning in a desert you'd be at a disadvantage.
Tiny amounts, enough to coat her fingernails. As far as rain goes the developers have confirmed it to be impossible or at least very difficult to implement as a water source. Meaning that for now physical sources are a necessity.
For balancing and the reason that we have loads of sources already (which my suggestion could potentially change) I don't think drawing water from the air should be allowed. If it -is- it should be a slow process over a few seconds to fill a bottle and be a utility "non-combat" skill.
Please us the "edit" feature if you feel the need to add more immediately after posting, It just makes your ideas more easy to read in one place and also is much neater too.