Now may not be a GREAT time, but the probability is that it's better than any future time. You're more likely to end up with more money in the future by investing now than by waiting. So unless you've perfected that time machine, it doesn't matter if now is a good time... it's the best time you've got. :)
Buy and hold, and not timing the market is a true and time tested method. But that does not mean you should not assess whether the price of what you buy is unreasonably high. Just like you would not buy real estate at any price because "RE always goes up". A valuation of the assets you want to buy is always prudent. The market is currently very highly priced by any historic measure. The market is so high not just because of amazing corporate results, but also because of the Fed. With interest rates essentially at 0, people are buying just about anything that might return a profit. Rising interest rates alone are likely to depress the market as capital would go to bonds and CDs. Maybe the best time to buy stocks would be when the market has gone through an adjustment. Short term I don't know what the market will do, but in the medium term a significant adjustment is very likely. What's wrong with protecting your assets until the inevitable correction happens?
Because we don't know what it will do. Even if you're right that it's overvalued, who's to say we don't see a stagnant market for quite awhile, say, 1-2% annual growth until earnings catch up with prices. At that point it's no longer overvalued, but you're still waiting for a "crash" to happen, and you've missed out on years of (small) returns, dividends, etc.
A correction doesn't have to look like a crash. Even if it does, you're likely to lose out.
There were PLENTY of people in 2010 thinking the 2008 crash was a precursor to the REAL big one. They sat out, and sat out, and sat out, and missed TONS of gains. Say you do sit on the side while the market climbs 40%, then it crashes back half of that.. you're still buying in higher than you would be by today.
The point is, even if you think it's overvalued, sitting and waiting is usually a worse move anyways, because you just can't know, and can't time it. It seems like, on a gut level, it should work, but study after study shows this is just not the case.