I was just having this same conversation the other day with Matthew. Most photographers are simply not going to releases the copyright, but rather will write in a clause, that gives you usage rights. Some will release the copyright, but you can plan on paying much, much more. As Jerry stated you could hire someone who would be an actual employee of your company, and skirt this issue, as it would be a work for hire by default. The other thing you need to consider is that copyright exists at the moment of creation, so even if the contract states that the photog is giving you copyright, I'm fairly certain, you would also need to have a legal copyright release, in addition to the contract..
I know and realize a lot of folks don't understand the thinking of why photogs seem so protective, a couple simple examples would be something like this.. ( keep in mind as a photog myself, this doesn't mean I would subscribe to this, it would greatly depend on the situation )
Lets say I am hired to shoot pictures of a no-name garage band and I release the copyright to them...5 yrs later, they are huge, my image which they now own lands on the cover of Rolling Stone, which in turn allows the band to sell 5 million cd's, and the cd cover is also one of the images I shot..The band just made a boatload of cash from record sales, and Rolling Stone also paid a hefty price to feature that cover on millions of magazines. Since I turned over the copyright, I make zero, not do I even get a credit for the photo, which doesn't help my business.. yes a an extreme sample, but the thinking behind it is the same..
Another example would be you hire me to photo your wedding, and I release the copyright, after spending hours cropping , editing the images.. You know have these images, which you send out to 50 family members, who proceed into walmart to get discount prints. Normally those 50 family memebers would need to order prints through me the photog...so I just lost those sales.. secondly they go to Walmart to get discount prints, and now all of the skin tones have a greenish hue to them..so they get the print, proudly hang it in theor home..a visitor comes by, looks at the cheesy cheap greeninsh print, and is told, yeah Bob the photog shot those, and they say, "we won't be hiring this guy, your family looks like a bunch of greenish trolls...more lost revenue, because the photog released copyright...
and another exampe using a Getty stock image which actually happened to a client of mine. Sperry shoes licensed an image from Getty images for use on their website and print ads. Sperry in turn told my client he could use the image on his website and even emailed him the image..Eventually my client gets the nasty-gram...you see it's all about keeping control of the images..my client used the image to promote the products it sells, and naturally Sperry is happy with this, but Getty is not, as my client did not purchase the image, and Sperry doesn't get to spread the image around to retailers as they don't own the image, the retailers are making money from using the image, but the photog doesn't stand to make anything, unless Getty collects and passes it along.
It's kinda weird for me to sometimes be on both sides of the fence, with this whole copyright thing, but I think I get an advantage in learning and understanding the mindset a little better from both sides...and yeah most ( not all, but most ) photogs can be dick heads, plain and simple in terms of how they enforce their copyright. The ones that will succeed are the ones that will change and adapt to the indusrty.