Invincible, schminvincible. Sometimes, you just have to point out the obvious.
"The first thing is that I don't think it's possible [to go unbeaten all season]. The other thing is we are not playing for that. Because, if you play for that and that is one of your objectives, maybe you draw too many matches because you don't take risks to try and win matches."
"You are happy just to be undefeated. If you get 10 draws, you get 10 points. If you win five and lose five, you make 15 points. I'd prefer 15 points and lost matches than 10 points and be undefeated."
"But we aren't focusing on that. We don't. If against Tottenham we are 0-0 with 20 minutes to go, we are not going to play for the 0-0 to be undefeated after 21 matches. No way. We are going to play matches to win. And, as a result, sometimes we will lose."
-Jose Mourinho; source: Guardian
It's pretty simple, really. More points are better than fewer points. Three points are better than one point. Three points from two games are better than two points from two games. Winning just half your games will get you more points than drawing every single one of them, which could possibly get you relegated even. Sure, an undefeated title win is an easier thing to remember and throwing out INVINCIBLES is quite impressive in casual conversation. But Arsenal's 90 points from their "invincible" season wasn't even a record haul at the time (Manchester United held that honor with 92 from '93-94), and was eclipsed in each of Mourinho's first two seasons (95 and 91, respectively), too. United also collected 91 in 1999-2000 and matched the 90 in 2008-09.
Basically, winning the title by gaining as many points as possible (or at least as many as is needed to beat the team in second place) is the goal. The only goal. Duh.