Well, I sort of agree with that.
If the goal is the have him play LT, then I'd agree he should only practice that.
If the goal is to find out where he can make the most meaningful contributions to the team you'll have to run him through the different positions, evaluate each, then get him focusing on 1-2 roles from there.
The flaw with that logic is if he misses at LT, then the team is boned AND you may have stunted his development at a different position.
For me I look at college players as prospects, where you can project what they will do, but until they are on an NFL team taking snaps you don't really know what you have. To maximize their chance of success all rookies should get a chance to show what they have against NFL players, identify where they best fit and then start getting reps there to enhance their ability to make meaningful contributions.