If it's not money you'd miss I'd probably do it. Each ESPP period is less than a year so it's a slightly higher rate, annualized, than the 5%.
This is what's key, but the rate is more than slightly higher. If your holding period is 6 months, then the average amount of time a dollar is sequestered in the ESPP account before purchased is 3 months. Your first dollar is held for 6 months, but your last is held for zero months.
What does a 5% return over 3 months annualize to? 20%. You won't find a better no-risk return without shifting to an employer with a better ESPP, and even much more volatile options will usually underperform an ESPP with even this mediocre discount. Compare with CD rates.
Two real problems with an ESPP:
1) Trying to hold the stock leads to a catch-22: if you do that, and practice buy-and-hold, you'll wind up ridiculously overexposed to your employer. If you try to minimize your exposure to your employer by selling after a couple of years, your best case scenario is taking on tax overhead from extra taxable events. Better to just sell right away and diversify immediately. You'll owe taxes on the discount either way, and the benefit for holding for 2 years is really tiny. Just be sure to sell it right away every time.
2) The tax rules are needlessly complex. I don't know about your brokerage, but ours doesn't report cost basis correctly on 1099s. Even though we pay taxes on our discount as income, the cost basis winds up reflecting the discounted purchase price. So we need to unscrew that manually when filing to avoid getting double taxed on the discount. Usually just adjusting the cost basis in the obvious way is correct, but if there's a loss, then the stock price at the beginning (!) of the offering period can become relevant, even though it really shouldn't matter. Your HR folks should be able to provide you with some assistance, like historical quotes, and probably some limited guidance on reporting proceeds from disposed ESPP shares.
You are right that your upside is only $500/yr, but I still think it's worth it. You only have to learn how to navigate the paperwork once, and $500/yr adds up over the years.