The problem with this lately has been sharp declines causing people to hit the IRS limit, which is based on the FMV on the initial offering date, and thus end up not being able to purchase as much as they expected. So the effective return on the money they set aside is way lower, and they could have paused contributions to put that money to work elsewhere.

Example: Stock @ $100 on the offering date and you are able to contribute $10,000 for 6 months. Stock dips to $25 on the purchase date. With the IRS Limit of $25k, you are only able to purchase 250 shares max ($25k/$100). Thus with a 15% discount on $25, you'd only be able to buy $5312 worth (250 * 21.25) and get a refund of $4688. In total of cash and stock, you'd have $10,938, which is still a gain, but only half the gain you expected.

Obviously, this is somewhat of an extreme example, but not wholly unprecedented in the current markets. And as stock prices remain low, the IRS limit isn't as far away as you'd expect for some stocks.