For just travelling around Europe, I think it's overkill. I've visited quite a few places here just using English, and the only times I've used German instead of English were when I visited areas near the German and Austrian borders. Even then, English was still generally understood.
As for my motivations for learning German, well, I live and work here in Germany. You can do a lot with just English here, but I always had trouble getting paperwork done and with dealing with the various government offices. I swear that they must have hidden all of their English speaking staff from me back then because I never seemed to meet any of them. Now that I can speak German, though, I've met quite a few who are happy to chat away in English with me. (@_@)
You need German if you are going to live in a German speaking country and it is also useful if you work for a German company elsewhere. To me it seams that learning a language just for traveling would be a bit of an overkill anyway, and if that's your main motivation for language learning you are right, you can get by really well in Germany with only English.
Not a German speaker or learner (yet) but :
I think "practicality" may not be the best justification FOR or AGAINST studying a language. Take it from someone who's studying Italian AND teaching Filipino. I love Italian because of the sounds and beauty of the language, and my students are devoted to learn Filipino to have deeper connections to their heritage. None of the above are to just be "practical" . Question is : would YOU enjoy learning German? (speaking, listening to, reading German).
While it's true you can get away with English in many German-speaking countries, you'll get a richer experience at least understanding the basics and responding back. I wish I did. I was in in Swiss German speaking Switzerland this summer on a retreat and could have had more connections to the the people I met.
here's a funny, sassy video on the subject : https://www.youtube.com/watch?v=L0Delsxdc-c
good luck !