MAME is about accuracy, most of these "ports" are watered down older versions of MAME that go against what MAME is about.
Getting people interested in playing less accurate versions of MAME doesn't exactly help MAME at all. The last thing MAME needs is more people who just want to play games and don't care about accuracy.
MAME is about accuracy and preservation, once you take accuracy out of it to play it on a portable, you might as well buy one of the retro packs out for the DS / PSP. Plus the fact that such MAME ports might hurt sales of commercial packs like the Namco packs for the DS / PSP don't help either.