I'm a promoter and also advise promoters on a very infrequent basis.
Cat 5 races are actually good for promoters if they have a lot of time. By definition you can't give anything worth anything (money, merchandise) to 5's - so a no-prize list Cat 5 race is a requirement. The promoter collects money and keeps virtually all of it.
There is a 50 rider limit, but that's okay. With a lot of time, you have two Cat 5 races.
With 4/5 races, you're limited to 75 but now you're obligated to give prizes.
If you're time limited, then a 4/5 race might be the only way to have both the 4's and the 5's race.
The series I promote is designed for early season legs. It's geared towards 3's, 4's, and 5's. We short under 40 year old Cat 1-2's as they only have one race. Pros only have one race too. We know that our paying racers are Cat 3-5 for the most part, and we know that if given the opportunity, many of them will race twice. So we make it possible for this to happen.
The only way to change a promoter's mind is to have a lot of racers ask for something. It may be that the promoter, looking at the field sizes, thinks things are perfect, but it may not be the case.
For example, we always combined the M40/Women/Junior in one field. Last year we had a "new" race, and we tried two different fields - M30 and Womens (on alternating weeks). We got a lot of women and killed our Pro/1/2/3 field as the Cat 1-2 M30's chose one race or another. This year we had a separate women's race and it was very successful. It took three years to do this - one to think about it, another to try it, and a third to make it permanent.
Promoters ultimately need a decent ratio of racers per field. If you can convince the promoter they'll get enough racers to not lose money, the promoter will think about it.
a promoter and a racer,
cdr
*edit* I should add that a promoter that emphasized 4's and 5's has struggled for two years, even though he has a 3's race. Without the 2's and a second 3's race, it's difficult to attract teammates of the beginner racers.