it is likely that a directory placement in Yahoo will help your SERP results there too
You never know when they might start incorporating their directory listings into their algo
If I correctly remember (and this fuzzy head sometimes doesn't), when the SE launched, the folks at Y! stated that they of course spidered the sites in the directory, but inclusion in the directory in no way guaranteed inclusion in the Y SE index. Sites had to make it into the index on their own merits, and no boost in ranking came from a directory listing.
Of course, they never precluded changing that in the future.