I'm generally in favour of this idea, especially for the small to mid size businesses that don't necessarily need a full time dba or systems admin. And yes, you will always end up paying for support.
However, once you reach a certain scale, i think you are better served having a dedicated in-house resource to tune your servers.
As an example, I worked for an emerging telco until 2001. EMC took the IT director out for lunch and persuaded him to allow them to "trial" a Symmetrix with us. We had reviewed EMCs hardware and ruled them out for a couple of reasons, most notably that the actual disks and SCSI arrays internally were only SCSI rather than Ultra-Wide SCSI2, and we had horrendous throughput requirements.
To cut a long story short, we stress-tested the EMC with our application profile and succeeded in flooding the cache, which resulted in massively slower throughput. EMC were politely asked to come and remove their box.
The moral of this tedious story is that the vendor may not always understand your application profile as well as you do, and once you get to a sufficient size that you can have the expertise in-house, it generally pays to do so.
Where I see this kind of application device winning is in the medium sized (10-50 staff), non-IT based companies that have no requirement for a dedicated system admin or DBA. Software providers could offer a standard server build which they think is appropriate for their application.