Grayscale was founded in 2013 and is on a mission to invest in digital currencies. The firm just published a research paper on the 'investability' of digital assets. Here's the opening statement of the paper:

“It’s not every day, or even every decade, that an entirely new asset class is born. Yet, through a combination of computer science, cryptography, economics, and network theory, digital assets have arrived and are proving that they are an asset class unlike any other. As they transform our global financial infrastructure and challenge modern monetary theory, we believe digital assets are one of the most exciting investment opportunities of the 21st century.”

— Grayscale

Here are the takeaways:

In any portfolio, in the long run, diversification matters. Money managers work hard to find diverse assets that will provide optimal return/risk ratio to their clients (me, you, us).
Digital assets, such as cryptocurrencies (Bitcoin is ONE of them) represent a brand new investment opportunity that is uncorrelated to other asset classes. The important part is that it's uncorrelated – meaning it provides a return stream that enhances diversification.
Looking at numbers from December 31, 2016 through May 31, 2018, it appears that small allocations to Bitcoin can significantly enhance the returns of traditional portfolios without materially increasing volatility.
Looking at the same time frame, it seams that investment portfolios containing allocations to a mix of digital assets performed even better than those only including Bitcoin.

And here's the graph you need to see:

Note: 2016-2018 was a very particular period for digital assets. The whole space was booming. Fast – maybe a bit too fast. And depending on where you stand on the topic, you may want to consider that past performance is never a guarantee of future results.