Back when I was an amateur I paid a model to "help" with my portfolio, with the same thinking that "if she's a pro model, she's probably going to help me get better photos right?"
It was a complete fail. Not so much with her, but moreso the fact that I just wasn't capable of taking advantage of what she had to offer. I wasn't sure how to communicate my ideas effectively, my lighting sucked and was fairly generic, and my concepts weren't really concepts so much as they were just images I pulled online and wanted to copy. Even despite her help, it was just a royal waste of time. Simply put: if you're not at the level where you understand lighting, composition, directing and communicating with models, the coming up with and execution of concepts, etc etc....then you're not going to get anything terribly special. Yes, there's the oddball model that comes along once in a while that can make anyone look "good", regardless of their skill...but if you want to look "great" then you need to have a better grasp on all the aforementioned skills/qualities.
Since transitioning to a full time working photographer, and having worked with professional agency models for the last 5 or 6 years now...I often get this question about whether a photographer should consider hiring a pro model to "boost" their portfolio. My advice is always the same: if you need to pay a model to work with you, then you're not good enough to take advantage of what she can offer anyway and you're likely wasting money. I suppose if you were rich and could afford to book Cara Delevigne, a full team or hair, makeup, wardrobe, and probably a creative director....you'd probably end up with some sweet photos just based on the surrounding talent working with you. But most people don't have $50k to drop on a test.
Oh, and from what I've seen having hung out on set with photographers, and having ran a rental studio for 2 years: the majority of models who need to be paid to work with you might be friendly and all, but there always seems to be a hint of "this person is beneath me skill-level-wise". Sort of a "you're just a noob" air about them, and "I'm just going through the motions so I can pay my phone bill" attitude. It might not be noticeable to all, but I've caught it plenty of times watching shoots....and the difference is obvious compared to when someone shoots with someone equal to them who WANTS to be there. That of course in my opinion is a big part of getting good results: working with those that want to be there and are looking forward to collaborating with you. That's why my advice to newbies is to always test within your skill-level and slightly upwards, and work your way up the ladder. Trying to fly too close to the sun always ends up the same way.
Granted, these are all just broad generalizations based on personal observations...and yes, again...I'm sure it's possible to pay a model who is more experienced and get something usable. But I'm also pretty sure it'd be luck more than the model helping. Evey time I've had a photographer come to me for advice or an opinion on some shoot they paid "an awesome model who could help their book" I'm always disappointed with the results. 99% of the time they could have put that money into some nice wardrobe, and a trip somewhere or renting a nice hotel suite, and shot with another amateur model and gotten much better results.
Hone your skills, take your time, and slowly but surely you'll improve to the point where you can eventually work with those "better" models. If you work hard enough, they'll be contacting you to test and you'll be much better off having worked out all the kinks prior to shooting them. I compare it to buying a Porsche 918 Spyder or Ferrari Enzo, and entering a race at Laguna Seca, or competing in time trials at the Nurburgring....but you've only gotten your learner's permit a week prior. Get some experience, figure out how to really make the car work for you, and you'll be more competitive and satisfied with the results in the long run.