Yes, because design patents expire, there's even greater reason to need to defend them while they exist. Design patents and trade dress protection often go hand in hand. In order to show trade dress infringement, one needs to show consumer confusion between the two products, and IIRC one needs around 4 years of solid registration before protection is enabled. So when you don't have trade dress protection yet, you would have to use your enforce your design patents to ensure that consumers won't be confused for 4 years or so.
Design patents can be implemented before the release of a product, but trade dress protection is decided by the market after being available to consumers for a certain length of time.
A handbag designer like Gucci would enforce their designer handbags with design patents when they release new ones, then enforce them with trade dress later on.
It's still not correct to say there is an obligation to defend design patents. That term implies a certain thing--that what you are obligated to defend will be diluted if you don't do so.
The patent itself will not become diluted, thus using the term "obligation to defend" is incorrect. It may be a good idea to use it to help establish trade dress protection, but your design patent will last just as long and still be enforceable even if you choose not to defend it.
Design patents can be implemented before the release of a product, but trade dress protection is decided by the market after being available to consumers for a certain length of time.
A handbag designer like Gucci would enforce their designer handbags with design patents when they release new ones, then enforce them with trade dress later on.