I've recently been though the same thought process as you.
When it comes down to it, there are two choices:
1. Use the grain. You may get a good result, albeit the chances are low due to potential for moisture having got in there, or other shit / bugs getting in there. If it's no good, you've wasted 4 hours on brew day, 2 weeks fermenting and time cold/bottle/keg conditioning. Plus there's the heartache of having to empty full bottles or your keg. Not to mention the $20 of yeast and hops.
2. Feed the old stuff to the birds and fork out $15 for 5kg of new grain.
For me the choice was easy. better still, if you've got kids they'll enjoy watching the birds hook in to the old grain.