The more I think about it, the more I think signing Alzner to a big, long-term contract would be a terrible move.
Since people here tend to be hostile towards any sort of analytical argument, I decided to just look at stats most people can agree on: goals. Over large periods of time looking at goals is probably equally, if not more valid than looking at shot-attempts (Corsi) because the sample sizes become large enough that good and bad luck tend to even out. The results for Alzner are not good:
http://stats.hockeyanalysis.com/rat...5000&teamid=0&type=goals&sort=A60&sortdir=ASC
Sorting by various columns, we find that Alzner is:
129th of 152 in on-ice goals for/60 (amongst qualified defensemen)
27th of 152 in on-ice goals against/60
79th of 152 in on-ice goals % (exactly 50%)
151st of 152 in on-ice goals for/60 relative to teammates (i.e. do teammates score more or less while Alzner's on the ice vs. off the ice)
35th of 152 in on-ice goals against/60 relative to teammates
136th of 152 in on-ice goals % relative to teammates
So as expected, Alzner fares well on the defensive side of things. He limits goals against both in raw numbers and relative to his teammates. But he is absolutely crippling offensively, so much so that his goal differential on really good regular season Capitals teams (only missed the playoffs once during his time) is only 50% and relative to his teammates, his goal differential is one of the worst on the list. He's amongst such notables as Roman Polak, Hal Gill, Deryk Engelland, Kevin Bieksa, and other notable bad players.
I've thought that Matt Niskanen has helped improve the perception of Alzner, so I looked at his rankings and they are as follows:
60th (of 152)
19th
21st
81st
15th
31st
The bolded puts Niskanen near players such as Ryan Suter, Mike Green, Drew Doughty, TJ Brodie, and Duncan Keith.
So in terms purely of on-ice goal differential relative to his teammates, Niskanen has clearly been the better performer than Alzner over their careers.
Even if you choose to look at shot-based metrics rather than goal-based ones, corsi and expected goals paint a similar picture of Alzner: that he's pretty good defensively but absolutely deficient in the offensive end.
I can get behind keeping Alzner for his PKing abilities and supposed leadership at like $4M, but I can't get behind paying him a huge sum of money for a long period of time when the results at even strength just haven't been there throughout his career. He is deployed in a more defensive role than average, but so are players like Vlasic, Tanev, Hjalmarsson, Klein, Braun, Hamhuis, etc. and they are paid nowhere near what Alzner's rumored to receive and they all perform better than him in terms of goal differential.
Just because the Capitals currently have no one better in the system to replace him doesn't mean they should overpay him. There will be other options out there (either via trade or UFA, especially with the looming expansion draft) and that's why it's especially important for them to both see Alzner apart from Niskanen and to also see what else they have in their young defensemen.