I don't know how much it costs to add the no expiration feature. But, my answer would be that it's only worth adding it on if it's cheaper than buying a new PH for the number of days you have left. Here's an example with made up numbers:
Expiration Add On - $150
One Day Park Hopper - $100
Two Day Park Hopper - $194
Based on that, if I only had one day left on my original PH I wouldn't add-on the no expiration feature, since it'd be cheaper to just get a new one day PH. But, if I had two days left, I would add on the no expiration feature, since it's cost is cheaper than a new two day ticket.
Remember though, I have no idea how much the no expiration cost actually is. I have a feeling that it's definitely less than $100, which means it would always be beneficial to add on the feature. Make sense???