Other mistake: When the velociraptor jumps for Ian, it hits an electric panel and shorts it out (the red lights go off). In the aerial shot of Kelly's acrobatics the panel lights are on and the raptor is gone. When it jumps from the ground to the catwalk the lights are off again. Then on again when Ian and Kelly run from the shed. (01:36:00 - 01:36:48)
Other mistake: Immediately after the T-Rex breaks out of the facility into San Diego there is a panning shot into a cabin room, which is filled with people, none of whom are showing any reaction to the T-Rex. (01:47:10)
Other mistake: When they have the baby Rex and the parents approach for the infant, there's an outside shot of the T-Rex roaring as Ian moves away to the other side. After he does that the Rex on the other side silently roars, then that same roar bit is repeated from inside the trailer in the very next shot, but the roar is added in, showing repeated takes.
Answer: It's basically about facing one's own mortality. Many humans feel that they 'feel the most alive' when facing (and overcoming) dangerous situations, the more challenging, the better. Roland is a big game hunter, to him, the ultimate challenge would be to hunt the biggest and (presumably) most dangerous predator ever to exist. Facing the danger of the T. Rex would make him feel better and mightier than he had ever felt in his life.
Twotall