When a guy ignores your texts but still asks you out, he is most likely keeping you around because it is convenient for him.
He takes forever to reply to your texts or ignores them altogether even when you know that he is online and obviously available.
This clearly shows that he has very little interest in truly getting to know you and allowing you to get to know him.
He continues to ask you out because he may enjoy the fact that he has someone that he can call his girlfriend.
This may be a huge boost to his ego, especially whenever he is around his friends.
However, in ignoring your texts or taking forever to reply to them, he is clearly showing that he is not truly invested in building a stronger bond with you.
Unfortunately, a guy who acts in this way will keep you around for as long as it is convenient for him.
However, once someone else comes along that captures his attention, it is often easy for him to leave you for that new interest.