ILB is not about seeing into the backfield, its more about reading pre snap keys in the offensive line, identifying plays, and reacting quickly without false steps, all of which Fletcher does well. Height is an advantage but can also be a disadvantage. Height usually means you can carry more weight, which helps an ILB with durability over the course of the season taking on blocks as is common in the 3-4 alignment. It can also mean you have longer arms so you can disengage from blockers more effectively. But, it can also be a disadvantage, because taller ILB/MLBs lose the leverage battle more often than desirable vs. the run. That is why Urlacher is not a premier run defender, and maybe why BB made Vrabel the Will and not the Mike when he moved him to ILB after Seau's injury. A tall Will can be a more intimidating pass defender to a QB because he can get to more balls and create more difficult throwing angles, provided the LBer is agile enough to get adequate drops in coverage. That is Urlacher's strength. So Fletcher has less than ideal height, but he had at least 12 passes defensed, has proved durable for 10 years, is an experienced, emotional, team player, very coachable, excels at pre snap reads, and rarely takes a false step. BB would adjust the defense to suit his strengths and hide his weaknesses. Bruschi is listed at 6'1'' but his actual height may even be a shade under 6'.
