1. Yes. The GPS I use most for cycling is a much older Garmin model - the eMap that I've had for over 10 years. Although not marketed for cycling it suits me better than most of their newer models.
2. Not often at all, but seeing my position on the map does help me see just where I am and therefore how much farther it is to the next stop, town, etc.
3. No, but sometimes as a way to find a shortcut back if the weather turns bad or something else comes up.
4. They're most helpful when touring since they include the locations of businesses like grocery stores, restaurants, motels, etc.
5/6. I purchased Garmin's streetmaps but also downloaded some free topographic maps. I use the latter mainly for hiking. When I got the Garmin maps (2001), CD was the only option. CD/DVD has the advantage that you can combine maps from various sources - so I can have both the street and topo maps in my GPS and choose which to see at any time. I also use the maps on my PC at home for trip planning purposes. OTOH, Garmin's maps on CD/DVD are protected and tied to a particular GPS serial number so they won't work (at least without some hacking) on another GPS you might buy later. The SD card maps can be transferred easily between Garmin GPS units.