The amount of people that push this is astounding to me. Yes, it's historically been a valid strategy if your horizon is 30 years plus, but there's not many that can reliably say they won't need to tap their assets over that period. There are spans of time in the not so recent past that would have you losing so much money that DCA and hold might delay your first home purchase more than 10 years. Or if you had to unexpectedly raise a kid (happens to the best of us) having to sell when down to meet care expenses. It's also true that past performance isn't indicative of future; and in my opinion this is more true now than ever. The structure of our economy, the global economy, the strength of the dollar, and political stability are all shifting. There are many scenarios that could put us into a long-term sideways market.
Outside of a 401k with retirement many years away or if you just so happen to be a lucky trust fund baby, I don't think it's a great idea. Life is likely to throw stuff at you where you'll want or need to mobilize your money and it would suck hard to miss those moments or struggle through them because the market isn't doing well.
In a way, the amount of people in the retail market that think they can just ignore risk due to long term trends is what has me the most spooked about current valuations. That's the kind of sentiment that makes especially nasty bubbles; when buyer sentiment is detached from risk valuations exceed rational and it doesn't take much to panic the herd. There's quite a few similarities to the current market and the dot com market. Look at what DCA'ing and holding would have done for you if you started in '97 and wanted to buy a house in 2010. Congratulations, your strategy would of had you renting an apartment until your 40.
Stocks are a great investment in all but also planning for major events in your life and adjusting your risk exposure accordingly matters. Blindly DCA and holding 100% something like QQQ is probably inappropriate for the vast majority of investors.