The question you need to ask is what a prime can give you that your zooms can't. You have every focal length between 16mm and 300mm covered, so primes in this range need to provide something your zooms can't to be worth owning. You own some terrific zooms that are razor sharp. Unless you are looking at a Zeiss Otus, you are not going to improve sharpness with a prime.
I own 2 of the 3 same zoom lenses you have (only different on the long end, I have the 70-200 2.8 II instead of the 70-300L). I own the following primes for the reasons listed:
35/2 IS - Smaller, lighter and 1-stop faster than my 24-70/2.8 II. The f/2 aperture and IS allow hand-held shots in really low light that are not possible without a tripod using my standard zoom.
100L Macro - Macro capability and a nice portrait lens. I often pair it with my 24-70 or 35mm for day trips. This is one of my most used lenses and as Marsu42 mentioned has a great fun factor.
135/2 - Awesome head shot or head/shoulders portrait lens. Also great for indoor sports, sharp and super fast AF. Incredible bokeh wide open. Worth owning in addition to my 70-200 2.8 due to its compact size, inconspicuous black color and extra stop of light.
I'm looking to add a 50mm prime. At this point I'm strongly considering a Sigma 50mm ART. I rented one for a week over the holidays and loved it. The only thing I don't like about it is that it's almost as large and heavy as my 24-70. I suppose I'm still hopeful that Canon will come out with a new 50/1.4 IS. If they come out with a 50mm prime as good as my 35/2 IS, I'd buy it in a heart beat. A fast 50 would give me better low light capability and ability to isolate my subject than my standard zoom. A Canon IS lens would also be smaller and lighter.
I'm also looking to buy a 300/2.8 II this year if I can every convince my wife that I really "need" it.