The reasoning of the folks who have argued that it's ok to run in thunderstorms is that 'well, very few people are hit by lightning, so I'll take that tiny risk'. They're banking on arithmetic like this: 300 people per year are hit by lightning in the US, and there are 300 million people in the US, so their 'odds' are 1 in a million of being hit. Unfortunately, this is numerology rather than sound statistics.
The problem with that approach is, almost none of those 300 million are in a place where they could be hit by lightning. Most people, most of the time, are indoors or inside a car -- places you should go to avoid getting hit by lightning. So, with no effort or attention, most people could not be hit even if there were a thunderstorm in the area.
Once there is a thunderstorm in the area, almost everybody heads for safety -- and everybody should. Go inside your house (and stay away from the pipes); if you are away from homes, go to your car; if that's too far or you didn't drive, head for low ground; if you're in Flatlandia (which is where I grew up) head away from anything tall -- away from trees, telephone poles, and especially away from metal towers. These steps are easy and almost everybody takes them.
The right statistical question is "Of the people who are in a position where they could be hit, what fraction do get hit." This figure is more like 1 in 3000, rather than 1 in 1,000,000. Almost everybody does, intentionally or accidentally, take suitable precautions. Be one of them.
Since most of us who run, do so for our health, I'll point out that being hit by lightning is not good for your health. A high fraction of people hit, are killed. Of those who do not die, a high fraction have permanent health problems.
Beyond the fact that it's bad for your health and easy to take appropriate precautions, is the fact that shifting your schedule a little will let you get your run in easily. You need to give a thunderstorm about a 30 minute clearance, wait for 30 minutes after the last thunder you hear. But, an individual thunderstorm is not a long-lived event. 90 minutes is long. Moving your run 2 hours only means go out at 7 instead of 5, or vice versa. If it's very important to get the run in, then moving it a couple of hours is easy enough, and enormously safer. If you're looking at a line of thunderstorms, or a supercell, it's a longer window and you may well have to miss the run. On the other hand, such systems also produce tornadoes and large hail and have severe winds such that it'd be hard to do a workout anyhow.
For more information about thunderstorm and lightning safety:
Red Cross on Thunderstorm and lightning preparedness
NWS Lightning Safety
NWS home page -- with current watch and warning areas
Added: from Hank Roberts' September 21, 2010 comment:
