There is a fair amount of implementation overhead for implementing both numeric addition and string concatenation with $add, and there are some related open tickets to fix: SERVER-6193, SERVER-6194.

Also I can see how '+' might be considered a convenience in javascript (though personally I would prefer a separate operator like '.'), but I'm not sure if there is a benefit in using the same $-expression to perform both addition and concatenation in the aggregation framework.

EDIT: $concat currently supports the following:

• All number types are converted to their string representation
• dates are converted to ISO-formatted strings
• code, symbol and string are already string types
• Null, undefined and missing fields ("nullish values") are converted to empty string
• timestamps are converted to ctime format + the increment portion. e.g., "Wed Jun 30 21:49:08 1993"
• all other types error out