The forecast has looked like that every trip I have been there, and not once did it ever rain for more than an hour. Dont sweat it! I promise!
Yeah, I used to live on the East Coast of Australia - it would be scorching hot, then rain for 20 minutes (and boy! did it rain!), then it would stop and you could see your footprints drying up behind you as you walked!
I looked at the 10 day as well and freaked for a second till i remembered it always says rain before i've headed down !!
Where is Conzumel in relation to Cancun (dont want to google it), friends are down there and they said it's raining, it stops and starts, some days sunny, some very cloudy.
It said the same thing before my trip in July and we ended up having beautiful weather, so keep your hopes up and enjoy your trip!!!