Instead of a store of value you could see Steem as a store of power or a kind of stock.
If you a buy a substantial stake in Steem you have the power to support the steem ecosystem, to support certain authors, artists, movie makers, etc. and thereby, you increase the incentive for these authors to publish their content on the steem network. This in return increases the value of your Steems.
Steem can also be seen as a kind of stock as you get dividends (=curation rewards) dependent on your activity on the network and (potential) capital gains.
In my opinion a store of value has to be expensive to be created. That's why I think that, in the cryptoverse, only Bitcoin (proof of work) can take that role.
This is exactly why I think it's only a matter of time before large corporations start investing in Steem (like they now all have social media departments)... purchasing Steem to basically power people's opinions or content ($1000 upvotes for whoever creates the best Pepsi video) and then power down when you need that money back, it's truly a win win situation. I absolute see scenarios where Political parties hit Steem hard every 4 years.