Presumably, the Office installer can modify/update the Microsoft `Silverlight.plugin` (which is installed in `/Library/Internet Plug-Ins/`), which Safari and Chrome could potentially be using.

It's primarily just a precaution against updating files which an application is actively using.

[EDIT]

After further investigation, it looks like it's actually the fact that the installer modifies the fonts in `/Library/Fonts/Microsoft/` which is the motivation behind requiring that several apps not be running. The following is an excerpt from the XML `Distribution` script that's in the `16807Office 2011 14.3.5 Update.pkg` installer package:


    <choice id="fonts" selected="true" start_enabled="false" start_selected="false" start_visible="false" title="fonts-title">
        <pkg-ref id="fonts">
            <must-close>
                <app id="com.apple.Safari"/>
                <app id="org.mozilla.firefox"/>
                <app id="com.google.Chrome"/>
                <app id="com.operasoftware.Opera"/>
                <app id="com.microsoft.Word"/>
                <app id="com.microsoft.Excel"/>
                <app id="com.microsoft.Query"/>
                <app id="com.microsoft.Powerpoint"/>
                <app id="com.microsoft.Outlook"/>
            </must-close>
        </pkg-ref>
    </choice>