Should Rondo play hard every game? Absolutely.
However, we all have to admit it's human nature to take mental days off from work every now and again, especially when we're well aware that it's not a particularly important day. This isn't me defending him. I think given what he's being paid he should supersede human nature. However, it shouldn't be a surprise for anyone that this happens with athletes.
To compound this problem, for the past four years Rondo has essentially "come into his own" in an environment where the regular season has had very little value. We've seen the whole team, including the Big Three, coast through very large parts of the season to "turn it on in the playoffs." And quite frankly, it's worked pretty well.
So it shouldn't be a shock to anyone to see Rondo following suit.
The good news is that with the depth this team has--assuming it's not absolutely ravished by injuries--players should be able to keep their minutes down and thus go all out for the minutes they do play.
And I can see this even with Rondo. I think Doc can keep him down around 34-35 mpg with the presence of Terry. As awesome as Rondo is, let's not forget some of the success we've had with great shooters like House at the 1 spot, given the way they open up the floor for everyone else. Terry should be able to do all that and more, which should comfort Doc enough to keep Rondo's minutes under control during the regular season.