Cell phones are everywhere. In some countries the average is more than one cell phone per person! Cell phones have stopped being used for just phone calls and are now full on multimedia and data storage devices, especially when you're talking about "smartphones." Yes, they carry the phone log and contacts list which may be very important to your case, but you must also consider the evidence found within text messages, picture messages, video messages, emails, website browsing history from the phones, GPS/location based services, calendars, audio files, Microsoft Office documents and more! All of this information could be crucial to your case and you need to call in forensic experts to capture all of it.
To give you an idea how close smartphones have become to fully functioning computers, the author of this website is currently writing this very web page from his phone while waiting for his flight at Newark Airport. Who knows what kind of data might be found on the smartphones in your case!