I'm not sure that managed service providers are 'increasingly popular' though I'm sure they'd like for that perception to get out there. The reality is that a company that opts to use this resource needs to understand that it's going to be very similar to posting an ad, but much more expensive. They're going to get a lot of paper, but few strong candidates because the best recruiters don't need to participate. They're busy with the clients that pay a premium for the quality service they provide. The recruiters that do participate are generally either newer recruiters who don't have a strong client base, or recruiters looking for another place to send candidates they don't have anything for. Those recruiters will not actually put time into recruiting on these jobs. Why? Because it's not worth their time. Especially when they can see that 50 or so other recruiters are working on the same req. They might be better off playing the lottery. The company then suffers because they think all these recruiters are 'working' on their job, but they're just checking to see if they have any resumes that are close and if so they'll send them and that's it.
Secondly, these providers discourage live contact with the companies, so you have no real sense of how hot the job is or what kind of pipeline is in place. It's just not a good use of time. Like a posting, the company might get lucky every now and then, but it's not something to count on.