You're right in that it's not going to double your miles (unless your carrier has customers that do just a snotload of HAZMAT freight). But worse, it won't pay you anything more than a regular load will, or at least not enough additionally to take on the added risk and the added hassles. It used to pay a lot more, 20-30% more, but sometimes double a regular load. Not any more, we just give that away now.
I used to regularly jump ahead of other vans on the board ahead of me, because I could haul HAZMAT, but I wouldn't go as far to say that it yielded me anywhere close to as much as 20% additional miles. I just got loaded out ahead of a few people, so maybe 5% additional miles.
Then I was with Panther for 5 years, and they wouldn't put placardable HAZMAT on a van at all, but at the same time the endorsement still got me some loads because they also wouldn't put non-pacardable hazardous material on a van if the driver didn't have the endorsement. Then I moved over to Load One where I did haul placardable loads, but only 5 of them in 4 years. Soooo, last year I dropped the endorsement. It just wasn't worth the hassle or the money to keep it, especially since so few customers are willing to pay anything above a regular rate for it.