Unlike the Koreans who do it for international marketing purposes, the Japanese inject English words in their songs just because they think it's cool. The same way westerners would get Chinese calligraphy for tattoos or entrepreneurs would name their restaurants with French or Italian words. :)

all of these are true, but keep in mind that alot of japanese words come directly from english, and they could be just speaking japanese, but with english borrowed words. but yes most of the time it is just cause they like the sound of english and they like to throw it in.

Yeah, as mentioned above, including English in songs - even if it makes absolutely no sense or it's used completely out of context - is seen as a cool little extra in Japan. Walking around, I've seen so many shirts and bags with strange English written on it, which often makes no sense. But, in Japan, it's cool! I once saw a Japanese song on TV with bits of French thrown in (I can't speak French so I don't know if it made sense or not!), but English is more commonly used. Similarly, though, Japanese people wonder what the fascination is with having <em>kanji</em> tattoos and accessories which, sometimes, have strange or offensive meanings... We think the written characters are cool :) 

こんな質問にすべての返信は「はい」なんです

In my opinion: They are facinated with the Western culture and therefore mixing English in their songs, would be the 'right' or as someone answered sound cool to use it.

I am studying Japanese and have noticed that there are few words in English that are used in their language, only the sounding of the words may not sound quiet English at first.