You've muddled orthogonal concepts together here.
1. Common shares are not worthless. In general, just ask any founder who's had a successful exit. Founder shares (unless purchased along side financial investors for hard cash) are always common shares; if your straw man were correct, then there would be no wealthy founders.
2. Liquidation preference is a negotiated term which does have a rational basis for existing. Whenever you're putting in a larger proportion of the company's cash than the ownership you're buying, it is crucial to have protection against someone essentially liquidating the company for the cash. Say investor X is putting in $8 into a company that has $2 in the bank, but X is only buying 10% of the company. If the company is liquidated for that $10 tomorrow, X gets back $1 and the common stockholders get $9. (There are other protections against a perverse liquidation incentive, too, but this is the economic one.)
3. The "real protection" from dilution is raising reasonable tranches of capital at a monotonically increasing series of valuations, which valuations actually correspond to a clearing price between bid and ask. Three key items here: "reasonable tranche," "monotonically increasing," and "clearing price."
a. Reasonable tranche: raise reasonable sized rounds, because huge rounds create weirdness (lopsided power, outsized compensatory "asks" by investors, etc.).
b. Monotonically increasing: needless to say, down rounds are the big dilution problems. Sometimes they happen because life isn't perfect and problems come up. Sometimes they happen because the company screwed up and raised too much at too high a valuation previously.
c. Clearing price: if the company and investor actually agree on the "true" valuation then it's easy. If they still try to force a deal where the company wants a crazy "optical" valuation that the investor doesn't really see, then you'll get layering-on of sweeteners to make the effective valuation much much lower than the notional, but all kinds of terrible side effects may accrue.