This is a small addon that adds an option to export only selected objects for every registered exporter. It also gives you an option to export the objects to a separate .blend file.
By default children of the selected objects are exported too (the “Include Children” option).
Besides the selected, you can also choose to export visible or all objects in the scene, and/or to limit export only to certain types of objects.
For PLY format, all the selected objects would be exported as a single mesh.
When installed, it will add a submenu “Selected” to the Export menu, where the available formats would be listed.
moth3r made me do it, and rking helped to generalize and publish it. Thanks go to them