Actually, in terms of stage time, the ensemble are on more than most of the principals.
Officially, the three leading roles are Valjean, Javert and Fantine. That is *official*, and is apparently done so that they're the only ones they have to pay extra. Javert is onstage for something like 37 minutes of the show. Fantine is on for less than that (officially I believe she doesn't have to be a bullet boy in act 2, it's just tradition and for something to do, though that may have changed).
Compare that with the rest of the cast who are onstage practically throughout in a myriad of different roles. And even those who are principals usually have ensemble tracks, so for example Enjolras is also a convict, a labourer, a factory worker, the captain, a judge, and sometimes, though not always, a drinker at the inn, then the other servant at the wedding.
As for clearly defined, while I would say it's generally agreed who is a principal and who is in the ensemble, I've seen just this week Gavroche mentioned as a principal by some people despite him not traditionally being classed as such. Even Grantaire sometimes gets that. Though in a show where basically everyone has a recogniseable named part with at least a solo line, it's harder to delineate such things, as compared to shows where you are cast as one of four "Ballet Dancers" or "Gaga Girls".
I chose to comment on it being an ensemble show, because that is precisely how I've heard it described by notable performers from the West End production. That is how it is apparently treated and that is certainly how critics have described it.
Indeed, I think devaluing the ensemble goes against the spirit of the show. It's Les Misérables, not Un Miserable after all.
Revolution: like Christmas come early only with more death
"Les Amis Fun Package - The Awesome Traits of Each"
"She's basically Enjolras meets Amy Pond"
Sings Stars "way better than Russel Crowe"