The point is that Vista let's you 'kinda' install the software without even asking for administrative permissions even if one is required. This means that 8 out of 10 regular users will deliver a porked installation for themselves and then harass the customer service about it for nothing.
I'm assuming you mean by installing in their user directory structure somewhere.
I'm sorry- but most users don't know how to turn on a computer, much less change the install path. Sorry, but it isn't a legitimate argument.
If you are purposely installing something, either it is:
A) Legit software
or
B) "Legit" software bundled with spyware
In the case of A- what the heck difference does it make?
In the case of B- Vista did everything it could to prevent you from installing that with system access... This is the point in which you need a computer administrator to prevent stupid users from installing crap like that by not giving out the Administrative password.