The typical Blender behavior when you have a couple of faces selected for example and you switch to Vertex mode, it will convert your selection to vertices. This has pros and cons. An alternative would be to store different selections for Vert, Edge and Faces, so that when you switch between them, your mesh selection will be whatever it was when you were last in that mode. So if you have selected faces A and B, and you switch to Vertex mode, you don’t have anything selected. Then you select for example Vertices 1, 2 and 5. You return to Face mode and you still have faces A and B selected because those were left selected when exiting Face mode. Then when you switch back to Vertex Mode, you now have Vertices 1, 2 and 5 selected, because those were left selected.
The reasoning for this is that sometimes the user wants to do something with an edge selection for example but after selecting all the edges they might realize that something else needs to be done with a vert selection. Now they have to do that thing in Vert mode and after switching back to edge mode they need to re-select all edges.
It should be optional whether the user wants to convert their selection between Vert, Edge and Face modes. The other option would be to remember whatever selection was in each mode. This is the default behavior in 3ds Max and the user can convert their selection between sub-object modes by holding control while switching.