We've always found it to be worth it - but we are a family of four. Due to the national park rules, we would EACH have to pay the entrance fee because we enter on separate vehicles!! (Hubby and son on a tandem only have to pay one) That means, even if it is only $10 per bike, we were paying $30 for that one park! Since the pass was $75, it made sense to buy it even if we only visited a couple of parks.
The other thing it helped with is that is good for ALL national parks, monuments, and recreation areas. A couple of times, the road we were on passed through a recreation area and we would have had to pay to use the road. The pass got us through free.
As the poster above said, check your route and see which parks you'll be visiting. Some charge and some don't so it's hard to say if it'll be worth it until you do the math.