Beek, hope I'm not getting too far off topic here.
When it comes to engine bays (and trunks for that matter), I've always considered them a "functional" area more than a "show" area. By that I mean they serve the purpose of housing the mechanical bits; an area for the vehicle's working parts, with all of their related fluids, lubes, electrical, lines, hoses, containers, hardware, etc. And they are covered/enclosed, not constantly open or exposed for regular display. Therefore I like them to be tidy, but not 'custom'. While I admire the show vehicles that hide everything in the engine bay to make them disappear, it does not make sense to me to do that on a street vehicle that is driven....even a nice one. So I'm with you guys, painting the inside of the bay (and trunk, etc) black, or even giving it a light coat of bed liner or another similar 'functional' finish, is perfectly fine to me. I prefer that to a color matched finish that shows every bit of dirt, oil, grease, road filth, blemish, etc. Same for the engine/trans and accessories; a bland color, natural metal finish, or similar is preferred to a lot of bright colors and chrome.