Cell coverage can be spotty along some shorelines and non-existent if far enough off shore.
Using a VHF gives paddlers access to Channel 16, which is monitored 24/7 by the USCG and commercial vessels. Channels 67 and 68, for example can be used for nonemergency communication betw. vessels, even two groups of kayakers. I have seen trip leaders on Lake Superior confer via their radios. They agree beforehand which nonemergency channel to use.
Equipment is no substitute for skills and judgement. But if you need to communicate status and position, a VHF is overwhelmingly better than a cell phone. Plus a VHF w. a waterproof spec of JIS 7 or 8 will work when submerged, a cell phone? not likely.
Here is a link to a quick overview of JIS ratings:http://www.suite101.com/content/submersible-vhf-radio-understanding-waterproof-ratings-a236128