I pretty much hated the character. If he was that integral to the story, he should have been brought up in the other movies. Just like a lot of characters in TDKR, I felt like he was there mainly so Nolan could end the movie with more "twists", and in the end, it made me feel like Batman wasn't nearly as special of a character like what we saw in BB and TDK. So now any Joe Schmo can be Batman, cause he's just a symbol? Eh...
I also loved the whole "I can't believe I shot someone with a gun! Guns are bad! I shall never use a gun, so I'll throw this one away" to moments later with "hey Gordon is in trouble. I better get my shotgun out".
This is the first time he ever meets Bruce, what would be the point of him being in the other movies?
He is not just any joeschmo, he's the perfect successor to Bruce's Batman, given his obvious keenness, and same drive for justice that led Bruce to create Batman, only without the traumatic moment that made that fight for justice also a quest for vengeance for Bruce. Blake will not have this flaw, making him an even better Batman. He has enough training as a policeman to not be a terrible Batman immediately, and most importantly, one of the main themes of the trilogy is that the training and technique is nothing, the will is everything.
He's also not just thrown in there for a twist, he's integral to allowing Bruce to move on and complete his mission of inspiring hope and giving Gotham a fresh start, while having a successor that can be what he couldn't. He was Batman because enjoyed doling out punishment, Blake will do it for the justice in itself.
Alot of the issues people have with the film are less problems with the films, but problems to them because it doesn't share their vision. Like the 8 years - people keep complaining that Batman would never do that. THIS Batman would, because he's sacrificing his enjoyment of being the Batman for the people. It's a choice, one that he clearly wants to remedy, but it would hurt his whole goal. Of course as soon as the occasion arises, he jumps immediately at the chance, as Bruce should. People complain that JGL's character was unnecessary or a bad version of Robin, when really the nod to Robin is just a nice touch for the fans, in that he's serving as the mentee role. He's not Robin, really, as he's meant to become Batman, and his name is not any of the names of the Robin from the comics.
However, none of those problems are really problems in filmmaking, just not what these people in particular wanted. The actual problems of the film were the giant leaps of faith Nolan expected from the audience (How does bruce get back from the middle east with no money or contacts? I guess cuz he's the goddman batman. Batman taking tons of time to burn a bat in a building? Logistically confusing, but metaphorically pretty awesome symbol of hope), which he barely gets away with at some points, and the Modine character, who HAD to be there to show Batman's trickle down effect of hope (he inspired Gordon to inspire Modine into standing up for justice in the face of certain death), but just felt out of place in the movie. JGL's 'intuitive' discovery that bruce was batman felt jarring when watching it, but can also be looked at like he really was meant to be his successor. And no, he doesn't have too much screen time, it serves the story very well, and really paints him as the perfect successor to Bruce.
All just my opinion, but it's a bit of a pain to see people calling things "problems with the film" when it's clear that they just don't share the same vision, despite Nolan making everything incredibly relevant and important to his characters, and still doing an amazing job bringing in elements from the comics and doing them justice. The way they used the pit to have Batman be reborn was a genius way of using the idea of Lazarus Pit, and also paralleling his fall as a child, where he now has to do it all himself without his father's help. Nolan was actually able to improve upon classic things from the comics and make them truly meaningful to Bruce. Still blows my mind.