Generally agree. And some employers don't even offer much of a reduction, such as only 5% off. And only after a longer waiting period, like 6 months.

On the tax aspect, I think the most tantalizing part of ESPPs is to get a quick return by selling right after getting the discounted shares. At which point, any positive ROI after short-term taxes is still free money. So while taxes depress the potential gains, it still yields a positive return. But I agree with you that instead of taxes, market movements can easily wipe the gains entirely, and that alone is reason enough to just not bother with 5% or even 10% ESPPs.