ANY sin is an ultimate offense against God. Just "because it's only a venial sin" doesn't mean that's it's any less an offense to His almighty dignity and goodness. In fact, venial sins make the soul sick, weaken it, and dispose it towards committing a mortal sin.
A venial sin will still bind you to the devil. Think of it this way: a venial sin is like a thread, and a mortal sin is like a chain connecting you to the devil. A single thread is easily snapped. But a thousand or a million or even more...they can be just as difficult to snap all at once as one mortal sin. In fact, it would take MORE work to loose the threads that bind you.
As Catholics, isn't it our duty to strive to be free from ALL sin, all inclination to sin, not just those that are mortal? Can't a soul still go to Hell with venial sins?