Ecchi series can be good as a series, or just as mere ecchi stuff.
So, depending on what the readers want to read, the current rating doesn't tell them if a series has a high rating because it's an overall excellent series (good art, good story, etc) with a few suggestive stuff, or a crappy series but with an especially good lewdness. Or if it's both excellent and extremely lewd.
Adding a separate "lewdness rating" would give a more accurate idea of what other readers actually liked in the series.
Moreover, it could even make it easier to know what is safe for children to read.
Series about armpit fetish can be suggestive, but it's not on the same scale than hardcore stuff.
So, depending on what the readers want to read, the current rating doesn't tell them if a series has a high rating because it's an overall excellent series (good art, good story, etc) with a few suggestive stuff, or a crappy series but with an especially good lewdness. Or if it's both excellent and extremely lewd.
Adding a separate "lewdness rating" would give a more accurate idea of what other readers actually liked in the series.
Moreover, it could even make it easier to know what is safe for children to read.
Series about armpit fetish can be suggestive, but it's not on the same scale than hardcore stuff.