Finding a lead by its Marketo lead id does have some real life use cases, so it would be great if the field could be used in smart lists. One example would be a database with a high percentage of duplicates, where list uploads require distinction between two records with the same email address and you would use a smart list identifying the exact record to add to the list.
In another, somewhat more obscure, use case I have an instance that is connected to a non-standard CRM where success criteria cannot be measured entirely straightforward with the Marketo data available. In CRM, success is measured at company level rather than person level, but the Marketo Lead Id that originated the lead is known. So therefore I would want to use a static list again to identify successful leads by their id.
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