Have you looked at the shares that crashed in the wake of the EU referendum and thought “ooh, I’d love to snap up some of those bargains, but I haven’t got the cash because I’m already fully invested?” I have. I’d have loved to have bought banks, insurers and housebuilders in the days following the vote — but my investing cash was already fully employed and I had to watch others enjoying the bargain hunt.

If you were fully invested in the FTSE 100 a year ago, you’d be up around 11% today, with dividends probably taking you to around 14%-15%.

But if you’d kept the cash and piled in during late June to early July, you could today be sitting on a 38% profit on Barclays, 54% from Aviva, and 34% with Taylor Wimpey. That’s just picking one from each of the three punished sectors, but other combinations would yield similar results. You’d be way ahead.

Which is really better?

So should you keep some of your cash reserved and ready for the next big crash? I say no, for a number of very good reasons.

The Brexit vote took us by surprise, but at least one thing we knew about it for sure was the date — you really could reserve your cash in advance and then snap up the bargains should the Leave camp win the day.

But when was the previous opportunity to have done the same? I’d say it was the banking crisis, but that started back in 2007. Banking shares didn’t bottom out for another two years — and we had no advance notification of the dates.

If you’d been watching Barclays back then, you’d have seen a slow and steady share price slide for all that time. Could you have called the bottom? You would only have had to miss it by a little bit either way for your strategy to have failed.

But the bigger long-term downside is that by sitting on cash and waiting for a crash, you’ll probably forfeit dividends for decades. Even if you get your timings spot on, you’ll probably only dive into shares around once every 10 years, and pull out of them again very quickly — and your timing is simply not going to be that good anyway.

Time is better than timing

Just think of all that lovely dividend cash you’d be turning your nose up at, taking a pittance in savings interest when there are solid shares out there offering 5%, 6% and more in annual yields.

Back to today, what would you do next had you timed the Brexit opportunity just right? Would you sell now and keep the cash for the next stock market dive? If not, when would you sell and how would you decide? And how long do you think it might be before your next boot-filling opportunity?

Couple all of that uncertainty with a decades-long lifetime of investing, and I reckon you’d be making a serious mistake keeping cash out of the market just because you’re hoping for a crash. It’s time that makes investing in shares the great long-term success it is, and you surely owe it to yourself to give your investments the longest time in the market that you can.

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Alan Oscroft owns shares of Aviva. The Motley Fool UK has recommended Barclays. We Fools don't all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors.