Tent for RTW Trip?

It would depend completely on the type of trip you were doing.

In the Arctic - yes , primarily for the mosquitoes.

For remote mountain trekking - yes for wind and rain protection.

Long term outback African camping - sure

Hiking the Appalachian trail - a must

For trips across Europe - probably not

In Asia going where most travelers go - probably not, guest houses are so cheap, some people do take along a mosquito net

If you have your own car or motorcycle it can be great on remote beaches and the extra weight is not a problem, for backpacking on the tourist trail it would be dead weight

I definitely wouldn't spend $400 on one!!!

I'd say bring it. I brought mine to Australia/New Zealand and used it a lot! Depends on the person!

The only time to take a tent is when you know you are going to use it most of the time and at a bare minimum 1/3 rd of the time, the places you are planning to go there is either a decided advantage to camping or virtually no other option but to camp (cycling the Tibetan Plateau, TransCanada or the Carretera Austral for instance). The reason to spend $400 is to have one of the lightest idiot proof tents in the world(yes I have one that complies with my requirement that I be able to put it up in the dark in the rain without any instructions on my first attempt because that's what usually happens). Be aware though that the safest tent in North America is Orange, Yellow or Red and the safest tents in the rest of the world are Green or Brown unless you are doing some high alpine climbing. Low profile is best.

Well, I paid USD10 for my tent (1.6 kg) in Poland and usually pay under euro15 for a 2 person tent weighing under 2 kg. Not paying more allows you to dump it if you no longer need it!