Assuming you meant "conspicuous", the problem with this idea is that when such officers want to participate in less than stellar behavior, they will obscure the numbers (by putting on a jacket, etc.). This could be justified to their superiors for any number of "tactical" reasons. After all, many cops regularly go on duty in unmarked cars or in plainclothes.

Cops are supposed to cite drivers for license plate covers and I see them out in NY the time--how can we expect them to enforce the display of their own numbers?

The only part of the uniform that police can't easily modify is the badge, which is also the part that unambiguously proves that they are police when necessary. (Not that most citizens would know how to authenticate the badge of their local police, much less those of all the federal LEAs [1]).

[1]: http://en.wikipedia.org/wiki/Category:Federal_law_enforcemen...