I was one of the poor bastards lining the street in the 1989 inauguration parade. I had been in for a whopping 5 months at the time, pretty much straight out of Marine Corps boot camp. At first, I thought it would be pretty cool. Then after about 2 seconds of preparation, I realized it was going to be the suck. On the day of the parade, we were on the buses sometime around 04:00 or so. They bused us up from Quantico and then we got into a formation for instructions. Stay at parade rest until Reagan's presidential limo drives by. Come to attention, render hand salute, then back to parade rest until the color guard and Bush come by. Then we marched out and got into our positions on the route. Holy shit, was it ever cold that day. We were on the parade route very early. I can't remember how long we stood there, but it seemed like 36 hours or so (yeah, exaggeration is my thing).
Yes, these types of things just suck for the service members involved.
Edited to add: I did get a real spiffy Certification of Appreciation with a stamped signature of some Army Brigadier General. That made it all worthwhile.