I agree with you that I am, for some reason unknown to even me, more likely to spend $20 in increments on things I will never use on iOS than I am to spend any amount on the Mac App Store.
On that note, however, Mac Apps do seem disproprotionally more expensive for similar functionality but that is a different topic.
Is that really still true? I know most apps I use have either required purchasing an update or have simply been replaced by OS functionality.
And there's something about committing to a Mac app when you're not sure if, in a week, one you like more will be available as opposed to iOS apps, which are basically disposable.
Of course all of this is tempered by my desire to support the products I use and the people who develop them.
On that note, however, Mac Apps do seem disproprotionally more expensive for similar functionality but that is a different topic.