So even if you observe 2 days of yom tov in general, you have to admit that 2 days of Shavuot makes even less sense. The basis for 2 first days of Pesach is that the messengers took a long time to get to the Diaspora and announce the date of the new moon, and if they left on the 1st of Nisan, they don't get there in time for the 15th, so two days are observed as yom tov to make sure one of them is the real 15th of Nisan.
You and I can disagree about whether it makes sense to maintain this practice today, when all future new moons have already been sanctified and we no longer rely on witnesses and messengers to set the calendar, and we have almost-instantaneous communication with Eretz Yisrael. And we can even agree on the value of minhag avoteinu (see Beitzah 4b), and you can follow the minhag of your ancestors who kept 2 days, while I'll follow the minhag of my ancestors who have been Reform for at least five generations. But in any case, we agree that two-day yom tov for Pesach and Sukkot made sense at some point in the past.
Shavuot has no date on the lunar calendar - it's just 50 days after Pesach (which always happens to be 6 Sivan on our current calendar). Maybe the messengers took more than 15 days to reach the Diaspora, but could they have taken 65 days?! Surely by then the correct date of Rosh Chodesh Nisan (and thus the correct date of Pesach, and the correct start of the omer count) was known, so there was one unambiguous date for Shavuot!
And you implicitly agree, if you only count one omer! If you think otherwise, then you should be saying each day "Today is the 27th or 28th day of the omer." (This isn't so far-fetched -- this is what you do during chol hamo'ed Sukkot for the Torah reading and the little blurb in musaf.) If there's only one omer count, then there's only one 50th day, and only one day of Shavuot. End of story.
That said, I acknowledge the practical benefits of two days of Shavuot -- if you're staying up all night and throwing off your circadian rhythms, you need another whole day to recover! I certainly could have used one; instead I woke up at sunrise this morning to catch the 6:19 train from Wingdale back into the city to go back to work, while all the 2-day people were sound asleep and fulfilling Psalm 149:5, with visions of cheesecake dancing in their heads. Needless to say, I was a walking zombie for much of the day.
Next in this series: Nothing in the foreseeable future! I observe 17 Tammuz and 9 Av. (Im tirtzeh hashem, they'll be feast days this year.) So someone will have to invent more holidays for me not to observe.
Update: Desh points out that he has beaten me to it. But he's wrong -- my explanation was not "much more thorough".