There are many instances when you are running one of the tools with multivalue inputs (merge, append, union) and you may not have the same number of input datasets each time. If you simply connect your (lets say 3) input datasets to merge but leave one of the inputs blank then merge won't run. You can get around this by connecting your inputs to the collect values tool and then passing those values on to merge. However, in order for this to work your downstream intermediate outputs need to be left blank. This means that each time you run your model in ModelBuilder that you would need to go in and set or unset the intermediate outputs. Ugh!! Furthermore, this precludes running your model as a tool. Double Ugh!!
I propose that Esri re-design the collect values tool to honor empty values. This could even be implemented as an optional check box. This would allow for more flexible models.
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