A lot of it depends on what is in the missing dump. If it's critical code, then the game won't run at all. If it's sound data, sometimes the game will work, but won't play any sounds. If it's palette information (often stored in PROMs), then the colors might be wrong. If the missing dump is some sort of protection chip internal ROM, then the game might not work at all, or it may be possible to "hack" around it (or simulate it) in the driver, depending on the complexity of the protection.
For example, in Pac-Man, the palette information is stored in color PROMs. In early versions of MAME, the colors were hard-coded. Eventually, the PROMs were dumped and the driver was changed to read the colors from the dump. The game still worked without the PROM dump, but it's more accurate emulation now. More info on color PROMs...
http://aarongiles.com/mamemem/part3.html
GroovyMAME support forum on BYOAC
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