It is the early days of bitcoin that we should be very much cautious about the advice of investing what you can lose into bitcoin. We can take such advice seriously for shits but for bitcoin, invest the amount you want to use for investment. If you have $10k and you want to invest $3k, invest it, even if you can't to lose it.
Your money is not going to loselt in bitcoin and we all know about it. So, what is the essence of investing only what you are able to lose?
Yeah Bitcoin is a very good investment and it has proven it self for years however one still need to be very careful in his or her Bitcoin investment because no one can tell what will happen in the future, I believe when Bitcoin was created in 2008 no one believes Bitcoin will be this big, the future is unknown.
Your advice can put a newbie into a big trouble, if he has $10k and he uses $3k that his not ready to lose like you said so maybe he will be needing the $3k in the next 1 month to come and for something very important for example paying of house rent etc. and he decided to follow your advice and then invested the $3k and when that time he will be needing the money starts coming closer a dip happens in Bitcoin what will become of him at that moment. So this are the reason will advise to use only your Discretionary income to accumulate Bitcoin so has not to get yourself into a financial mess.