Yes if you are earning only from bitcoins then you need to spend them regularly to take care of basic expenses, but if you have a day job then it will make easier to save your bitcoins for future.
So somehow it's really important to have a day job so that people's salary there will be used for daily and basic expenses.
With the status like that, their bitcoins are just saved and will just come out whenever they really need it.
It is a must to have a day job ifyou are just starting with bitcoin. It will give you lots of advantage if you have your day job and have your job that pay in bitcoin. One reason is that you don't have to worry if one of those job got a problem. Second, you will be enjoying some financial freedom because you are earning more than what you really need..