Being a 'selective' player is a VERY wrong approach to learning guitar ... you'll end up not being able to play whole tracks, and you'll lose coordination between different instruments (coz no one plays selective parts with backing tracks).. also afterwards when u play with a band you wont play according to drums tempo and you'll go off-beat more frequently.

maybe after you've learnt how to play well and have jammed with a band for so long then u can 'select' certain riffs from tracks and play them if u prefer.. but you shouldn't learn this way... also, the feel of being able to play whole tracks gives you confidence, feeling of accomplishment, and gets u motivated

Selective = n0t k00l
full = k00l

and in order to not be spamming here's why:
When playing certain/selective riffs from a song (let's take for example DT - Panic Attack <= the reason is the fixed time signature change in the middle-to-end 4/4 to 6/8) you'll be perfect in all of the parts by themselves and you'll always pause when switching riffs in the full song structure and that's not easy to solve when you've already practiced each riff by itself.

BUT if you're just practicing a riff you thought is cool to just test your skills then I don't think it's a bad idea to do that (but you'll still be n0t k00l)

I definitely have to learn an entire song. I have no real desire to just learn certain parts of songs, nor do I see sense in that. At least for me. lol

I tend to learn full songs but if it's really a difficult song I will be selective of the parts I can actually play at that point and work on that. I'm not gonna stop playing a song or riff I love just because I can't play the whole thing.

I don't tend to learn songs- at all, but when I do I pick the defficult riffs/cool ones so I can analyze them or just to improve my technique. I don't think that learning the whole song is necessary if your only looking to improve your technique.

I like to learn the whole song best but if there is just one riff in the song that I can't play then I don't stop playing the entire song. Or I skip that part or I do Improvisation which is also a good excercise anyway. It trains your ear as well as it increases your knowledge of songstructure and scales. Of course it is the most fun to play an entire song from front to back. Other songs just have one cool riff, with those songs I mostly just learn that riff and forget about the rest of the song. I wont learn a whole song if I just like one riff.