Excellent point. I believe it depends on the extent of the outage. Most times it seems to be fairly localised but one time my power came back quickly but I was not able to get DSL/Internet going for something like 15 minutes, so I guess that one affected the ISP area servers. But that time my backup Internet access (different ISP) came back really fast so I always have that option. If that fails too as long as the cellphone system works, I can tether my iPhone and use that. I have done it before, works fine, but this reminds me I better log the steps and keep it with my emergency numbers. Don't want to be trying to remember what to do at a crucial moment.
We use our cell phones for backup Inet access at work and I use mine for home backup Inet access as well. Since Inet access is crucial to work, we must have it or nothing gets done. I'm a firm believer in a UPS not only being there to keep a machine from being shut down unexpectedly (as stated) by also to keep from losing data. A router/hub/switch/cable modem will come back up just fine if the power in interrupted and the chance of something going wrong is minimal. A PC or server that suddenly loses power could be a whole different story all together. That's (IMO) the biggest part of having the UPS, to allow you time (I aim for at least 10 minutes of backup power) to save everything and shut it down correctly if needed. I'm sure you're well aware, don't plug your laser printer in to the UPS.