Diversification of your investments and your portfolio is one of the best thing you can do as an investor, 100% of the time. It's always, always better to not leave your entire portfolio in a single investment unless you're really ballsy and think that you'll be able to make money on that single investment. I couldn't imagine leaving all of my money in one or two investments, and chances are I'd age another 5 years from the stress of such a minutely diversified portfolio.
Diversification serves the purpose of minimizing a cumulative risk of a portfolio in case of risk events (through negative correlation between assets, hail to Markovitz), so it should be done exactly with this end in mind. But in the first place you should understand the risks involved with each asset that make up your portfolio. If you don't clearly see them and their interrelation, you may end up highly surprised when the whole portfolio starts losing value instead of just one asset. In short, diversification just for the sake of diversification may not turn out quite beneficial...
I would call such form of mixing investments within a portfolio a dumb diversification
