I think the advantage of the higher weight limit is the ability to accommodate bigger kids. I had a hard time when DS was small because he was so big - he's always been 95th percentile or higher, and there were points where we wanted to keep him rear-facing longer, or in a 5-point longer, but couldn't find a suitable, affordable seat that would allow us to do that. Only the very expensive/boutique brands were offering the extended weight limits then. He was 40lbs by his 3rd birthday and that was the upper limit on his 5-point harness, but I'd certainly have been more comfortable with a 5-point than with a belt-positioning seat at that age.
Now even the lower priced mass-market companies are offering higher weight limits. The seat my 5.5yo uses can be used as a 5 point harness to 65lbs. That would have been great to have with DS, but isn't a feature we'll make full use of for our 35lb 5yo. I think it is great that these seats are becoming so much more affordable, but I don't think that should be interpreted as a mandate to keep them in those seats up to the maximum weight/height limits without regard to age.