A DriDucks jacket is a good compromise for a light (and cheap) just-in-case rain shell. I have settled on heavy and light options for rain gear, leaning on the heavy option for my city/daily rain gear and the lighter one for hiking and compact travel.
I take a poncho for day hike CYA rain gear and shelter. It's easy to sit out a heavy shower sitting under the trees with the poncho over all.
As far as REI and the lifetime warranty/guaranty, if they sold it to you with that stipulation, they have to live up to it. Satisfaction is a different issue that failure of materials and workmanship. With the change in policy, I wonder of REI is going to go by purchase date?
From the current REI web page:
100% Satisfaction Guaranteed
We stand behind everything we sell. If you are not satisfied with your REI purchase, you can return it for a replacement or refund. Items must be returned within a year of purchase, except items purchased from REI-OUTLET which must be returned within 30 days of purchase.
REI’s guarantee doesn’t cover ordinary wear and tear or damage caused by improper use or accidents.
If your item has a manufacturing defect in its materials or workmanship, you can return it at any time. See our limited warranty.
If your item has a manufacturing defect in its materials or workmanship, you can return it at any time. Many of our items also have a separate warranty from the manufacturer, and you can also return any of those items that don’t meet the manufacturer's warranty.
This limited warranty gives you specific legal rights, and you may have other rights which vary from state to state.