Volvo S80 Remote Start: Immobilizer bypass decreases security?

Hi, I have some questions about the interaction between the Volvo
immobilizer & an aftermarket remote start system for a 2001 Volvo S80.

I was thinking of getting the Viper 5900 monitor & remote start. I
called the dealer to see if this was possible, and the head of service
there told me that it was. He told me that I'd need to bring in or buy
another key for the installation. I at first thought it was to program
the remote start clicker, but after a few questions, he told me that
it was for the immobilizer - that this extra key goes under the
dashboard.

I don't know much about this, so after a few minutes, I started
thinking that maybe it lessens the security of the car to have a key
or the chip from the key under the dashboard that can bypasss the
immobilizer? Or, is this just a technical neccessity that I don't need
to worry about, because it's being able to bypass the immobilizer is
contingent on the clicker turning the car on, and having a key in the
ingition? Or is the only real decrease in car security exist between
the time that I turn the car on with the remote start to when I
actually get in the car (assuming no one's hiding in the bushes,
waiting for me to get to the car!).

In all seriousness, I'd really appreciate if someone could please
explain this to me, and let me know if this would in fact diminish the
car's already good security system. I figure that in a worst case
scenario, I would just get the Viper unit that is just the monitor,
not the remote start. But I sure would love that remote start, too!

Thanks a ton for any help you can give me!!