Practice maturity is a measure of management not performance
Explaining the basic CMM 5-level model today I was reminded of a common misunderstanding about maturity: it measures how sophisticated or smart or advanced you are at managing a practice or process; it doesn't measure how well you are doing it.
If you are now huffing and puffing, let me explain:
You might only be at maturity 2 right now, but you are doing a really great job of it. Every transaction is done the same way each time and done to a high quality standard (or to a low quality low cost standard if that's what the bosses want - whatever, you could be doing it well, meeting KPIs).
In a few years you might document and entrench the practice and get measurements in place and implement a CSI program and get to maturity level 5 and deliver crap. You'll know it is crap from the dashboard and reports, and your CSI plan will have lots to be done.
Maturity measures how much formalism and infrastructure is wrapped around your procedure in order to make it predictable and manageable and improvable. Probably your delivered quality improves with maturity, but there are no guarantees. Maturity is not a performance measure.
People forget this. They think anyone at maturity 5 must deliver good stuff, and anyone at maturity 1 or 2 must do a poor job. Not so. Just like an ISO9000 certification, a high maturity proves nothing about how well the job is done.
Made in New Zealand 
Comments
Faith vs. works
Faith vs. works, performance vs. maturity... I'm beginning to think it's a theological discussion. Do good works suffice? Or are they simply a sign of grace? Does performance follow maturity? Or is maturity simply a sign of high performance?
Shall we fight for thirty years over it?
Charles T. Betz
http://www.erp4it.com
Overstated Benefits
You're correct, but "maturity" is of even more limited value.
First, maturity assumes that there is a one right way to improve the process. Two level 5's would both be following similarly refined, articulated, and defined processes. All of the measures that I have seen define levels in a context independent fashion. This is a weak conceptual foundation.
But, it is a handy metric. Management can examine its current maturity level and ask how much, if any, improvement should be set as a goal. That's practically handy.
Even granting its practical utility, the word "maturity" has the wrong denotation, and connotation. More "maturity" is almost always seen as being better. Moving up an IT maturity scale, however, is not always a good thing. As we get more (IT) mature, we lock in ever more strongly to refined, articulated, and defined processes. In our chaotic world, these sophisticated (and often rigid) processes can become shackles that bind us to processes that worked in the past, but may not work in the future. As you pointed out, more maturity may not be cost-effective. More maturity may also be a bad strategic direction in which to move.
repeatable process is anti-evolutionary
Made me smile. Real ITSM argues the same thing: repeatable process is anti-evolutionary. It is like a monoculture crop: most efficient right now, least resilient to sudden change
Potential and Execution
I really like the way you have put this into perspective.
The way I understand is that process maturity is only an indicator of the potential of a process within the IT and does not translate into how well the execution is being done. A process might be rated high in process documentation, or integration or governance, however the process execution compliance may be far from doing what is to be done the way its documented!
see what they are really assessing
yes, and if it is a CMM-style maturity we are talking about then you have to look at the actual assessment criteria to see what they are really assessing. If they have used the repeatable-defined-managed-optimised model then all it shows is that you measure and (try to) refine the process. It doesn't say anything about governance, or even quality of the deliverables (that depends on WHAT you measure).
And of course process maturity only measures one leg of the three-legged-stool: People Practices and Things. It says nothing about how good the people are (culture, knowldge, skills) or how good the tools are (efficiency, effectiveness).
most of all process maturity is an inward facing metric - it is unaffected by what the customer thinks or receives