invest when you have cash?

I hear some of the market experts saying, it is futile to time the market, so invest when you have cash, and sell when you need cash. I find this not only unwarranted, but also an incomplete advice. 

How can someone invest when stocks are priced at unprecedented levels compared to their intrinsic value? Do they mean to say, you have cash now, so buy Tesla? or Nestle? How about Valeant, or Rcom? I can list several stocks; but the point is, unless you have carried out some analysis of the business and its value, you cannot decide on buys or sells. It is fundamental to investing that you compare value with price. It will prove stupid if you go on a buying spree just because you have cash. You will have to wait for too long if you want prices at your levels, is again a remarkably unremarkable thought. It is better if we give these advices a pass.

There is only one occasion, and it is a powerful one, when you have to ignore market prices, and yeah, invest when you have cash. That is when you are investing in the market itself. That is say, S&P-500 index or Nifty-50 index. The index has to be a diversified market index, not a sectoral or thematic one. Three things matter then: Lower expense ratios; Higher liquidity; and lower tracking errors. Once the index is selected, go and invest when you have cash. In fact, invest periodically irrespective of the market prices. You don't have to compare intrinsic values and so forth. Over a long period, you will earn returns close to the market itself, which will not be too bad compared to other alternative investments available to you. This strategy is most suited to those who have no time or interest in analyzing stocks. 

I wouldn't recommend this strategy to any mutual fund investors making periodic contributions (systematic investment plans), especially for the US investors. Mutual fund managers can turn out to be idiots; and handing cash to idiots is as idiotic. In India, I would reluctantly approve since there are some mutual funds who can beat the market. Yet, this isn't my highly recommended strategy. I don't like to give cash to others to manage; therefore, I wouldn't advice other investors either. 

So you have two choices: Invest in the index irrespective of market prices, and carry on with your life; have fun. or 

Learn to play the value-price game, and pick stocks. Here you could have occasions when you have to sit on lots of cash. Invest when you have cash is a bullshit advice for stock pickers. They pick stocks when prices are cheap compared to value, and sell when otherwise.