Photoshop and Illustrator are forever open on my computer, and yet I am a big proponent of doing as much design as possible directly in PowerPoint. Very often, adding an effect or editing an image in PowerPoint is actually quicker than doing the same in Photoshop. And even more importantly, effects created natively in PowerPoint are almost always non-destructive, which means adjustments are far easier as presentation content continually shifts (because it always does…)
Gradient Boxes
One of my favorite techniques in PowerPoint is to place a semi-transparent gradient box over full-page imagery. This is a way of “editing” the photo to make it fade out on an edge or to reduce the opacity over a part of the image and to allow for the placement of text on top of it.
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Although I do agree that I wouldn't rely on Photoshop too much for presentation purposes, I do feel that it is a great tool to achieve everything that you described.
To make layers non-destructive, all you have to do is right-click on the layer (in the layer panel) and choose "Convert to Smart Object". Now you can apply all sorts of effects and even resize it without actually modifying it – you then simply remove the effects to start over.
Also, if you double click the layer, you can apply gradients and vignettes very easily using Layer Styles. Double click the layer, click on the Gradient Overlay option to turn it on, and choose Multiply as your blend mode. Whatever's black will be opaque and whatever's white will be transparent. Then you just modify the opacity for desired effect. Also, if you turn on inner shadow, you can set the Distance to 0 and increase the size in order to create a vignette. And all of these layer styles are non-destructive as well, so you can go back to turn them on or off at any time.
But in the end, you still have to save out a JPG for ever slide so it's definitely up to the user whether the desired look is worth the effort.