This is also a fun exercise to drive home my point (hopefully).
This website is a fun website to play around with. I have preloaded FTBFX and FSHBX into the link. Intermediate and short bond funds, respectively. Both actively managed with durations of about 6 and 1.5 years, respectively.
If you take the bar at the bottom of the graph and expand it to be about 1.5 years and slide the bar back and fourth, you will find that the short-term fund almost always had a positive yield for that 1.5 year period (with the exception of 2008). Also note that the intermediate fund (FTBFX) had several occurrences of not yielding as much as the short-term fund (FSHBX). It also had had a couple periods of losing value over 1.5 years.
Now expand the bar at the bottom of the graph to be about 6 years and slide the bar back and fourth - what do you see? You should see that longer term bonds almost always outperform shorter bonds.
Unfortunately I couldn't find a prime MMF in this particular tool which would be a closer representation to a MMA than a short term bond, but a short term bond fund is close enough to make the point.
ETA:
Here is another link comparing the following vanguard funds:
VBLTX long bond index - 14.7 year duration
VBILX intermediate bond index - 6.3 year duration
VBIRX short bond index - 2.7 year duration
VBTLX total bond index - 6.0 year duration
It's interesting to do a comparison of these as well.
It's also interesting to compare FTBFX and FSHBX - actively managed funds - as well.