I'm going to expand a little on what Nathan mentioned. Even though your YDNA put you in a specific haplogroup, that only represents a teenie bit of what's you. For example, suppose a Yemeni orphan was indeed adopted by Germans in the 1400s. That orphan was then raised as German and married a German, then their kid married a German and so forth, and by the time it gets to you all that Yemeni DNA is pretty much gone except for what's left in your Y chromosome.
To put it closer to home, we don't come from just one haplogroup. For example, I've done Y-DNA and am predicted to be R1b1a2. Taking that further, my grandfather on my father's side would be the same. However, if I were to test my grandfather on my mother's side, I know based on her background that it would be different (I plan to test an uncle to confirm that). Is his contribution to "me" any less than my father's father? And of course, let's not forget the mtDNA haplogroups. You can take this analogy further and further back, and with the U.S. being such a melting pot there is no telling how many different haplogroups you came from. That doesn't mean the test is useless. It's actually very useful on the "macro" level in tracing name origins, population movements and so forth, and as far as genealogy in finding surname matches and so forth.
Nathan - I guess my male line was more prolific than yours, or at least more inclined to take DNA tests. I have over 1000 exact matches at 12 markers, and 30 or so at 25!