Webroot and existing AV already installed

Hello.
I am just about to install my trial version of Webroot SecureAnywhere Complete.
I have been using Norton for years (actually NIS), but I am no more happy with it. Last year I tried Bitdefender and Kaspersky, but I had some problems with them, so I stuck with Norton.
All these AV's are incompatible with each other and require careful uninstallation of the competing product before installing.
I am thus surprised that WSA doesn't care about the presence of other AV programs and that you don't recommend uninstalling other 3rd party security programs. Isn't it?
Even if WSA is compatible with them, what is the purpose of having 2 (or more?!) AV programs installed?
In any case, given that I intend to use only one AV, possibly WSA, if it will confirm to be light but nevertheless effective, as you claim, what is the best way to proceed? Install WSA and uninstall NIS later or vice versa?
Thanks to whom of you may help.
Best regards,
--- Glauco