Currently the Target Release field is a custom field and as such does not scale well when many different products start wanting to us it. Aside from the fact that it is currently global and visible to all products, if we start creating a new one for each product named for example "<product> Target Release" then the boolean charts list starts to get really long as well. Keeping it as a single custom field to share between many products (bug 588602) also is not good as the list of values will become very long and confusing with all of the various versions needed.
Alot of the core code would need to be touched to implement this which is partly why we did it as a custom field instead in the beginning. But the benefit would be greater than the work required.
The JIRA migration is also requiring something like this to map to their Fix For field which is configured per product as well.
*** This bug has been marked as a duplicate of bug 584956 ***