Now that it's hard to get decent interest from other cash savings, I don't think premium bonds are so bad. We should all aim to have some spare cash saved, that you can access quickly without penalty or risk, so they're probably as good a place as any and certainly no worse than many savings accounts paying virtually no interest.
Yes, if you 'only' have a few thousand or below, chances are that you'll win little or nothing, but the average payout rate is 1.4%, which is higher than any other instant access savings account. The closer you are to the maximum of £50k, the more likely you are to get close to this rate, so the risk is quite low, and you can get your money back in a few days.
You're effectively gambling the interest you would definitely receive on your savings account, so it's up to the individual how they feel about taking that risk.
Say you have £10k of savings. The best instant access savings account pays 1.2%, so you'll get £120 per year in interest.
You'd have to get 5 x £25 PB prizes in a year to beat this rate, but typically, you'd probably get 2 or 3 prizes per year, so an effective interest rate of 0.5-0.75%, so less, but better than a lot of savings accounts out there. Plus there's a small chance you'll win big, and a reasonable chance you'll beat the best savings account.
I have built up £14k PBs over the last year. In 2019, my prizes equated to an interest rate of 2.1% so better than instant access savings. This year, I've only had 1 £25 prize so far, so obviously losing out on a bit of interest, but there's still the rest of the year to go.