1. I make heavy use of my someday maybe lists. During my weekly review, I move a lot of projects back to S/M if I don't expect to make any progress on them in the next week or two. That keeps my action lists short, so then I'm willing to look at multiple context lists when I have some time available. It also helps reduce overwhelm at the number of tasks you could possibly do at any one time, which can otherwise be an issue if there aren't limiting factors.
2. I still use contexts, and a few of those are limitations (gardening prefers dry weather!), but most are used for batching tasks. If I picked an errand off my task list as my next action, I'd like to know what other errands I could also knock off while I'm out. If I'm using a particular computer program, are there other tasks I could knock off while it's open.