Someone above cited 2.1 procs per minute, which sounds about right to me. That means, on average, 1 proc every 30 seconds. The Strength bonus from the proc lasts for 15 seconds, so for half of every minute, you'll have +100 Strength. So really, it's just a matter of comparison for you: would you rather have a static +15 Strength, or rely on the proc to give you +100 Strength for half the time?