"Marijuana is stronger today than it was back in the 70's, so it is too dangerous to be legal."
This argument is dishonest on many different levels. It is at once untrue, misleading, irrelevant, and a counterargument to prohibition.
It is untrue that highly potent strains of cannabis are a new thing: they have always been around, just not at their current level of distribution. Back in the 70's the supply was not as reliable and Mexican shwag (not good quality) weed was all most people had. Yet, there were already tales circulating about the "White Widow","Maui Wowie", "Northern Lights", "Purple Haze," etc. From ancient times, people have used hashish, resin scraped or extracted from the plant. Hashish is almost pure THC and other active chemicals. What prohibitionists call "skunk" is much less concentrated than hashish which has always been used safely by humans.
The argument is misleading, in that it ignores the fact that when a drug is stronger, people tend to take less of it. To imply that a cannabis user uses the same amount of potent cannabis as they would of weak cannabis, is like saying drinkers of vodka will drink the same volume of liquid as beer drinkers. Stronger cannabis is less unhealthful to smoke, because less volume is needed, so one's body doesn't get as much tar or other carcinogens.
It would be irrelevant if cannabis is stronger today, since there is no fatal dose of cannabis, and even the strongest cannabis is relatively safe compared to hundreds, perhaps thousands of legally-traded substances. For instance, any strength of cannabis would still be safer to smoke than legal "Spice" or "K2", which are pot-substitutes that popped up on the market as the result of pot's prohibition, and which, though entirely legal, has been responsible for at least a few deaths, allegedly many more. Of course no one makes a fuss about "extra-strength" pain relievers in pharmacies, despite the many deaths caused annually by analgesics.
Finally, the little bit of truth that was buried in the original claim: that distribution of potent cannabis has increased, is a great argument against prohibition. So, you're saying, we've fought against cannabis for 40 years, and now it's even stronger and more widely distributed? Prohibition laws motivate dealers to sell more concentrated product, since smaller packages are easier to transport, so any increased concentration and wider distribution are a direct result from the drug war.