1) How common is it to have the additional extensions to include that data for analysis?

2) I have seen flow tools that show the entire AS path. Are they just cherry picking which platforms they showcase for the best marketing, or are they enriching the data they receive from "lesser" platforms from an outside source?

For that purpose, knowing what ASes your data goes to is useful. It's even more useful to find an upstream network that includes a bunch of those.