If there is enough free space in the 1st partition to get all the content of the 2nd partition, that's right.
It sounds good, but I think there would be some functional problems, mainly in w!n installations. The real problems have to do not with the merging itself, but with the "side effects":
A problem that might occur is the limit of the filename path of 260 characters in the w!n filesystems. Adding another step in the path could make some files unusable.
What would happen with paths and disk names (D:, E:, F: etc) in the w!n registry and the installed software paths and files? Can such a function be smart enough to adjust all these points? Perhaps yes, but what if special administrator rights are needed to perform such tasks? I expect more problems with v!st@. So, I'm afraid that users without experience would be forced to reinstall some of their software. So, I think that users could merge w!n partitions at their own risk
With Linux partitions, I would expect far fewer problems that can be arranged by the configuration files.
But a general question is what happens if the 2 partitions to merge have not the same filesystem, or what happens if the new merged partition falls outside the filesystem limits (i.e. maximum size of a FAT partition, long filenames, unicode character names ... ). The user must be aware of the possible issues and the solutions applied by the program.
A final issue: If the 1st partition can't contain all the 2nd one's files, the merging can be done by steps, but this would be perhaps very slow, as it happens sometimes with slight partition moving.
*** It is highly recommended to backup any important files before doing resize/move operations. ***