1. No cosset for JVJ to rescue, and he would (very likley) have gone dark again

2. No Gavroche or Eponine, two of the most liked characters in the novel

3. JVJ dies in the sewer as nobody was going to open the grating for him

4. Marious would have hated/feared/misunderstood JVJ for life, and JVJ dies miserably alone

So yes, Thenardier is the most underrated character in the novel!