Bob, the fact of the matter is mate, you can only have so many of these players in your side if you want to be a success. You can't have a team full of "job doers". You need versitility, quality and the ability to adapt to different stituations within the match at all times.
Carragher is what he is, he's limited. Its as simple as that. You can't have a team of limited players otherwise you ain't gonna win all. Balance is important and Carragher gives us that, but we still need quality in there with him.
You absoloute 100% must have both. Look at Carragher for England. He's like a fish out of water. Look at Ferdinand, he's nearly always one of Englands best performers in tournements and regardless of what the clueless people on here say is the main reason Uniteds defence is what it is.
Carragher is very good at what he does and he plays in a position where you can get away with that. Just as Pennant does, but neither of them are what you would call class footballers. Agger although being a good player isn't any better than Carragher and it shows and out of the two, I know who's style I prefere and which one bring the most to the team.
The fact is you need quality in there. Quality and balance = success. Its as simple as that. Liverpool this season have had a distinct lack of both at times.
In reference to your last bit, Liverpool IMO havent lacked quality and balance at the back especially the two center halves. Infact I think its a testiment to Agger, that one, he's kept Sami out of the majority of games this season. Two, we have still kept a damn good record for conceeding goals still. And three, those to previous facts arent lies.
I understand Agger isnt your typical big burley centre half, who isnt as dominant in every aspect of his defending as Sami is. He personally took me a while to appreciate because of this but like I said to you before what he hasnt got now he can get later when time goes on. If you know so much about football you'll admit that this is one position that you get better with, with experiance. Like I said to you, most of what he needs to learn is mental i.e position, concertration, reading the game etc. On the ball, technically he's better than Carra, thats not saying much but I think these two over the next couple of seasons should compliment eachother perfectly.
Put it this way, Centre half is the least of our worries, its up top where we need "quality" or even where your man Pennant is.