There are two main concerns with moving into a self-created solution - standardization and support.
Standardization
Dell may load a lot of bloat-ware on their consumer level machines but for corporations or the Government the Dell X-Image process can be used to standardize the base level install. This process allows for an IT department to build a base OS standard environment applications on a single platform of hardware regardless of the equipment in use. After baselining the system the image is uploaded via a standard web interface on which you also select the hardware platforms being used in the environment - laptop, workstation, desktop, thin client, etc. Dell then takes your baseline and codes it back using their X-Image process encorporating in all necessary drivers for the models you have selected then sends it back to you. It's basically an outsourced slipstream of the OS made extremely easy by Dell. The nicest part if you are a corporation or Government entity? It's free. Contact your Dell sales rep and ask about the process. This allows for standardization not only for hardware via the same manufacturer but also for the OS and applicaitons in use on those platforms.
Support
Dell may be a P.I.T.A. for consumer level services but for corporate or Government they are right there with HP and other high-end channels. Next day replacement part shipment - or within 4 hours based on purchased support for servers - means sites do not need to keep on-hand stock of components except for maybe a few key resources for critical systems. There is also no need to train your support team on how to replace these parts as Dell will send a technician to your site with the part to perform the replacement, test the system following replacement, and take the bad part with them for return to Dell. Considering the cost of the support of the systems is built into the purchase price of the system the overhead support cost is lowered and the staff is allowed to focus on the more "fun" issues related to using Microsoft products in a large scale environment. IF you wish to have in-house parts and repair capabilities, Dell will supply on-site sparing of parts and offer training to your IT staff to perform the actual replacement of parts. The best part here? If your staff is trained and completes the warranty work in house, Dell sends you back a credit of x number of dollars per "call" - I put x as the last time I was involved in contract negotiation was 5 years ago and while it was $40 per incident at that time I would expect there has been some change in amoount.
I work for the Government as well and Dell while in my opinion as an IT professional is annoying and bothersome to say the least, and while I know I could build a better, more powerful, and more robust hardware platform for the same cost, in an environment where standardization, quick support, compatibility, and operational state of my users are all at a premium desire of the customer, I say leave the headache of those messes to upper management and Dell. It may cost up to $250 more per system to have them supplied by Dell but considering standard rate of a technician and taking into account the amount of time needing to be spent on building and deploying, then training and support, combined in with overhead costs for maintaining parts and stock, the cost difference is a loss not a gain.