Assuming people in general approach recommendations in the same way, LinkedIn recommendations are good at identifying strengths of a person, but not weaknesses. In that aspect, they're not total junk.
Obviously, I wouldn't base hiring decisions on LinkedIn recommendations, just like I won't base such decisions on letter references, but they can be a small factor that influences the overall decision (after an on-site interview for example).
That said, I'd say they are certainly junk, just not necessarily for the reasons stated.
As I see it, LinkedIn makes junk of itself by being way too cluttered with the products of non-stop demands to engage in all sorts of empty actions. LinkedIn is an ugly rolodex, not a network.
There are two sources of "signal" in this case: The content of the recommendation as well as the very existence of a recommendation. Not recommending people only addresses the first.
Being that the person requesting controls whether the recommendation is published, we shouldn't expect to see much negative. A well-written recommendation (even on LinkedIn), written by someone with some level of industry credibility, can actually be a fairly powerful tool to get noticed.
Now, though, everyone's polluting the channels with nonsense. Why would I care that Bob has 37 endorsements for "APIs"? That doesn't mean anything.
Besides, social proof is for malakas. If anyone turned me down because I didn't have enough endorsements on a website, I'd laugh that person off the fucking bricks.
I'm trying to figure out how to advance past senior developer myself.
If you want to take the management path, you should have some kind of management title by 32 and Director by 36.