If you’re purchasing shares at a discount that you can immediately sell at full price, that is effectively free money.
Whether you want to hold those shares over a longer term is more complicated. 1) do you think the company is going to be more successful than its competitors? 2) do you think the industry as a whole is healthy and growing? 3) are you comfortable having savings and job dependent on the same organization? (i.e.: if the company has a big loss, you may lose your job and the value of your stock/savings will go down at the same time)
If you’re not comfortable answering the first two questions, then you may want to consider buying the discounted shares, selling them immediately, and putting the proceeds into some kind of diversified index fund. Index funds are popular because they diversify around the losses (and successes) of individual companies and individual industries, and bank on the general phenomenon of long-term economic growth. i.e.: population increase and new products.
ESPPs are a great way for the company to get employees to care about share price and get emotionally invested in the success of the company. They’re a great way for companies to provide additional compensation to employees (and the executives who put the plans in place) without having to call it salary, which often has tax benefits for both the company and the employee.