I don't think you're going to find any specific technical advisory to NOT place the units as described. Mostly the "not good" suggestion would be in the areas of ergonomics, esoteric considerations, serviceability, and user training.
As to ergonomics the general placement of the monitor on top of the desk unit makes for a better viewing angle and user access to the cpu mounted devices.
As to serviceability - the vertical mounting can complicate the ability to service the unit, especially when the "mountain" of personalization (book,papers,reports,etc.) is built around and especially on top of the unit. This can significantly restrict the air flow and can lead to increased system failure. Also the units while being placed in thenon normal position can be jarred and some internal connection points can be loosened. Not common but it does depend on who does what and how they handle it.
User training to make sure that those users that have vertical standing, and those thatwant to, is necessary so that they clearly understand how to use all the parts in that position and the extra steps to take. Those CD's flopping around can be a support tech annoyance to the extreme.
The esoterics include are that they just lookbetter, "more impressive" if they are desktop with the monitor on top. Certainly footprint is a consideration, as well as all the additional things that people have. Also, there might be a problem with the unit becoming a poster board, again the air flow being blocked. Also it depends on how you turn them as to a pleasing visual appearance.
As long as there is good airflow, service access and user understanding of using devices you should not have a technical problem. The key then would be to explore further the "preference" being expressed and supporting the Vertical mount with how it improves productivity. That one always wins out.
Good luck.
