At today's On-chain Scaling conference, fluffypony gave an excellent talk about the importance and types of dynamic blocksizes.

https://www.youtube.com/watch?v=mM2ra-LzMQk&feature=youtu.be

Monero has successfully used a dynamic blocksize for over two years, proving its resiliency and ability to respond nimbly by surviving DDOS attacks.

Dash has no plan for a dynamic blocksize; hardforking is required every single time it is adjusted up or down.