There is very little reason to buy a small amount of Steem Power other than long term speculation. A $100 purchase of Steem Power is almost meaningless for curation rewards, voting power etc.
Imagine if only users with $20 of Steem Power could show an avatar on SteemIt.
Users with $50 could benefit from Reddit-gold style perks like highlighting new posts since you last viewed the thread.
Perks would help cement the idea that Steem Power is a form of social status in this community. It's not enforced by the Blockchain, but sites based on Steem would encourage stakeholding by providing these little perks.