The Watchtower believes that if you study the Bible alone, without their literature you will fall into spiritual darkness within two years. Here's what they say.
'We all know people who have spent days and weeks and years in study of the Bible and have learned little or nothing. ... Furthermore, not only do we find that people cannot see the divine plan in studying the Bible by itself, but we see, also, that if anyone lays the SCRIPTURE STUDIES aside, even after he has used them, ... and goes to the Bible alone, ... our experience shows that within two years he goes into darkness. On the other hand, if he had merely read the SCRIPTURE STUDIES with their references, and not read a page of the Bible, ... he would have the light of the Scriptures.' Watchtower magazine, Sept 15, 1910, p 298.
'Jehovah God has also provided his visible organization, his “faithful and discreet slave,” made up of spirit-anointed ones, to help Christians in all nations to understand and apply properly the Bible in their lives. Unless we are in touch with this channel of communication that God is using, we will not progress along the road to life, no matter how much Bible reading we do.' Watchtower magazine Dec 1, 1981, p 27, par 4 
1. If you study the Bible with Jehovah's Witnesses, you will not be studying the Bible. You will be studying the beliefs of Jehovah's Witnesses and their unique interpretation of the Bible. You will be studying how to become a JW.
Even though they can't prove it with any Bible, Jehovah's Witnesses believe they are the only 'true religion' that understands the Bible correctly because the Watchtower is 'God's organization'. They believe the Governing Body of Jehovah's Witnesses is God's only channel of communication to the entire world. Thus they believe everything the organization says, whether it is supported by the Bible or not.
Even though they might say that you can 'just study the Bible' and you don't need to read Watchtower literature, you will find this is not true. e.g. if you show them a scripture that contradicts what they believe, they refer back to Watchtower literature to assure you their beliefs are correct because they are 'God's organization'.
Jehovah's Witnesses typically have very little time for Bible reading and they don't see the need to do it. All their time is spent studying Watchtower literature. And since the Watchtower usually refers to Bible verses to convince people they have 'the truth', Jehovah's Witnesses assume they are 'studying the Bible'.
The fact of the matter is you can use the Bible to prove virtually anything. That's why there are so many different religions.