So a card like SPG, is it worth keeping open to accrue SPG points or is it better to close and churn?
The same question can be asked about pretty much every card since they each have different features that make them good (eg sapphire for international, united for baggage, etc)
This is all JMHO and may not be the case for everyone in all scenarios.
A card which has a 12 month wait time, is one i would close down and wait for the next bonus, unless the benefit is something i truly value.
A lot of cards don't have a set wait time, or they are shorter, making it easier to churn and keep the benefits.
A lot of people are churning under multiple SS#'s (spouse or someone else) which makes churning the "12 mont wait cards" not bad.
For example, the SPG card. One can get the bonus and keep it for a period of time and then apply for it under another SS #. This way you ca get the benefits of the card, and the signup bonus, sooner than a year.