He's 33 years old, a full 3 years older than LT. He had catastrophic knee injuries several times earlier in his career. At his very peak, he was a great talent but not at the same level as even LT's sub-peak. Now, take a look at his stats from his 31, 32, and 33-year-old seasons (2006, 2007, and 2008). The Jaguars had Jones-Drew around splitting carries with him, and Fred Taylor put up very strong numbers and went to his first career pro bowl.
Considering the fact that LT is, on the whole, a better talent than Fred Taylor, his career trajectory and late-career usage gives me a whole lot of hope for LT's ability to produce plenty over the remainder of his current contract length. If it's possible to decrease his cap number with a restructuring to allow Sproles to play the Jones-Drew to LT's Taylor for the next couple, then even better.