I find this a very controversial topic. In one hand covering others songs is definitely a good thing since one can learn lots of new techniques and ideas. But that's just the good part and many teachers or music students will only look at the good things that come with covering others songs. But let's talk about the negative aspects of covering songs.
I personally think that covering others songs can kill or to some extent ruin ones creativity. Suppose you are a big fan of slash and you have covered all his songs, studied his compositional ideas. Trust me, consciously or subconsciously this will impact your creativity and your style of music. It will be highly influenced by slash, even if you are not consciously trying to copy him. There is only one Slash, so your music will always be titled as slash influenced and it will never get its unique identity.
On the other hand if you minimize covering other songs and try to develop your own ideas, own skills, techniques that you need for your music then in the long run you will create a style of yours that many people will admire and perhaps try to incorporate in their playing. I think that's the ultimate goal of any musician. To be able to create something that's unique and that will live for a long time. So yes I stand against covering songs as a mean of learning guitar or any instrument as a matter of fact if you are willing to be a composer and performer. But if your goal is solely to be a performer then that's something that you must do.
