It really depends on many factors. Are we talking about a retirement account, or are we talking about discretionary investments? Is your portfolio large, or small?

For a retirement account, I would never invest in Bitcoin. It is much too volatile and a person nearing retirement age cannot afford the losses that volatility might bring. If we are talking about discretionary investments and a large portfolio, I see no reason why Bitcoin could not be a speculative investment in a well diversified portfolio. For most people however Bitcoin is too volatile.

For your average investor, I recommend investing in an index fund until they build a portfolio large enough for diversification. Then, and only then, I see no reason not to use Bitcoin is a speculative investment.

There is nothing wrong with speculative investments as long as you have a well balanced portfolio. Not only will it give you a chance to cash in on a bonanza, should it occur. It also adds a little bit of fun to an otherwise dreary portfolio.