No survival hunter will be let into any self-respecting guild.
20 second duration with 15% increase
20 * 1.15 = 23 seconds
30 second cooldown
30 - 23 = 7 seconds of ZOMG no double.
23 + 5 = 28
30 - 28 = 2 second of ZOMG NO DOUBLE.
Conclusion: You have to pop a 5 minute cooldown to trap someone for < 28 seconds WITH diminishing returns.