I suggest the possibility to have both built in IntelliSense and VA IntelliSense enabled each with different key assigments. This suggestion came from my past experiences when in the same project in some situations VA IntelliSense was not able to provide any suggestions while MS IntelliSense worked well. In the other situations MS IntelliSense failed bud VA IntelliSense worked fine. In these cases I dreamed of having Ctrl+Space for VA IntelliSense and Ctrl+J for MS IntelliSense so I could try one and then the other suggestions. The same applies for other IntelliSense functions.
The option you suggested enabled will use MS IntelliSense and if it fail, VA IntelliSense is used. I want to be in control of what IntelliSense version is executed by assigning different keys.
To be more concrete I wanted to be able to specify something like next lines in the Keyboard mapping: Edit.CompleteWord - Ctrl+Space Edit.ListMembers - Ctrl+J Edit.QuickInfo - Ctrl+K, Ctrl+I Edit.ParameterInfo - Ctrl+Shift+Space
And similarly for the VAssistX: VAssistX.CompleteWord - Alt+Space VAssistX.ListMembers - Alt+J VAssistX.QuickInfo - Alt+K, Alt+I VAssistX.ParameterInfo - Alt+Shift+Space
This way I'm able to use both engines if I want. VAssist can still replace the IntelliSense by simple re-assigment of keys in default instalations. My personal opinion is that VAssist's IntelliSense could be enabled this way and not somewhere "behind the scene" inside the VS.NET (but this might be architecture driven constraint).