Something big happened on 6th December 2017.
Largest crypto-mining marketplace, Nicehash was hacked.
A message on their website and official Facebook page showed that:
One of the reasons why hardware wallet is important to keep your Bitcoin offline if you treat it as one of the ways to store the value in your investment portfolio.
When you keep it online, be it an exchange, a mining pool, online wallet etc, it is prone to hacking activities.
This is a robbery in the digital world.
Given the rising value of Bitcoin, security measures must keep up with time. While you may want to keep some coins for daily use (Bitcoin is a medium of exchange as well) such as buying a Durian, the store of value part of it should be put offline. Keep it in a hardware wallet and treat it just like a gold bar (well, it is more expensive than gold now given its current market price of USD13,000)
When something is valuable, treat it like a valuable.
Sidenote:
This would be a high profile news over the crypto world. Hardware wallet will sell like hotcakes. As such, the tradeable and usable supply of Bitcoin will keep declining because more units are being shifted offline by those who treat it as a store of value.
A logic in supply and demand theory will keep Bitcoin at a high price level.
Though keep in mind about the possible effect of Bitcoin Future markets:
Cash markets, such as Bitcoin, are there to serve investors, but a futures market is something different, it is there for people to hedge against risk. It is a pessimistic way to invest, and it bases itself on profiting when things go bad. ~ Darryn Pollock
https://cointelegraph.com/news/big-fish-entering-bitcoin-waters-all-eyes-on-december-10-and-18