Aftermarket strut tower bars are generally considered to be decorative on many brand new cars. However, it seems to be a bit more logical that a chassis with 100K miles would have developed some flex and could use a cross-member to stiffen things back up.
Background: the specific car in question is a 2004 Subaru Impreza WRX wagon, daily driver with 110K miles, none of them off road or racing.
So, the specific questions:
Does an aftermarket strut tower bar have any measurable effect during an enthusiastic daily commute?
If so, how do you measure the effect?
I have to admit, there have been plenty of times when I have wished that there was a bar across the engine bar just to give me something to lean on (as opposed to the intercooler!). That doesn't seem like a sufficient motivator, though.
EDIT to follow-up on some of the points in the answers: the fundamentals of the suspension have already been sorted: Hotchkis sway bars with new bushings, KYB Excel struts (that are effectively OEM replacements) with Kartboy endlinks. This isn't an autocross car but the easy targets have already been addressed. Now we're literally talking about tightening up an aging chassis.
And the answer can be no, there won't be a difference.