Way to point out the few plays that mean nothing to the conversation. Nice job, sure, but expecting your LB to make plays in the backfield all the time is entirely unrealistic. Let's look at some of your other replays...
http://goallineblitz.com/game/replay.pl?game_id=225805&pbp_id=4986193Do you expect your LB to to chase down that WR? No, that's your CB/safety's job which he does here and your LB is not expected to do. That 90 speed still isn't going to chase down that WR so there's no need to have it. What greater agility does gives you though is the ability to break on the ball better. Could 70 speed/80 agility have had a better chance of catching that WR before the first down marker? I bet you it would have had a better chance than 90/60.
http://goallineblitz.com/game/replay.pl?game_id=225805&pbp_id=4987049If the RDE didn't make this tackle, your increased speed/diminished agility probably wouldn't have made this tackle. (Here is an example of too much speed and not enough agility; watch the FS:
http://goallineblitz.com/game/replay.pl?game_id=225462&pbp_id=5650573 )
http://goallineblitz.com/game/replay.pl?game_id=225805&pbp_id=4987851Here's an unfortunate special teams play where your LB is held up on two other LB's blocking you. Since an LB typically won't have high blocking, it's kinda embarrassing that your LB isn't able to shed these blockers to try and make the tackle.
http://goallineblitz.com/game/replay.pl?game_id=225805&pbp_id=4988023Another example of a teammate covering up a potential gaffe by your player. The HB cuts to his left and if Ryan Hunter isn't there to make the tackle, there's a very good chance your LB wouldn't have been able to backpedal in time to make that tackle.
http://goallineblitz.com/game/replay.pl?game_id=225805&pbp_id=4988865Held up on the line by a LG already blocking a DT. Congratulations, all the extra speed just means you get blocked that much faster instead of avoiding the block.
http://goallineblitz.com/game/replay.pl?game_id=225805&pbp_id=4989528Here's a blitz where the HB blocks your LB cold. Could you have shed that block with higher agility?
http://goallineblitz.com/game/replay.pl?game_id=225805&pbp_id=4992143Nice play here, but it's not like you covered that much distance. In short distances, agility is king. Again, the matter at hand isn't if your LB is going to catch that 90 spd WR; it's if he can make the adjustments to cover the HB/TE/slot WR.
Case in point:
http://goallineblitz.com/game/replay.pl?game_id=225805&pbp_id=4993180Your AI has you blitzing here, but you break the block and go after the ballcarrier, leading to a missed tackle. With higher agility, you break the block faster, you make a faster cut, you accelerate out of the cut faster, then you're in a better position to cover a short-yardage tackle. If you make that tackle, he doesn't get a first down. You tell me how higher speed benefits you here.
If you're going to present an argument, you need to show all sides.