Tebow has a competitive demeanor, is committed to football, has some athletic ability and has shown he can make some plays, mostly running with the ball.
Tebow has not shown he has enough skills that there is a position or role on an NFL team that he can be effective at.
I see 2 possibilities here.
1) Tebow is the right guy mentality, personality and commitment wise to be a Patriot QB, and BB brought him in to see if there is any chance the actual skills needed can be coached and developed. In this case, I would assume the plan is that he goes through at least 2, if not 3 training camps and seasons before ever being considered a viable option on the field.
2) We don't even carry a QB3 normally, so if we can find a 'good teammate' that has the versatility to be the emergency backup at 4-5 positions and could fill in on special teams, why not see how the final 53 shakes out and see if there is a spot, much like we would use for a Niko Koutivides type player if there is room.
In my opinion it all depends on how much they thought of Mike Kafka. If Kafka wasn't getting the playbook very much or simply had a lack of talent as a QB in this system or otherwise, then bringing in an upgrade at the QB3 position makes some sense. After all, Tebow can surely offer more of a versatile skill set at other areas than Mike Kafka could, no?
Not only can it potentially be an upgrade in that area, but it may also allow an extra player to be kept on the 53 man roster who normally wouldn't have made it. I have no idea what Belichick's plans for Tebow are, but the possibility does exist that we can now cut down on a wedge blocking reserve LB who wouldn't see the field, a TE5 (although I thought the chances of that were pretty low in my opinion), or even some other kind of special teams player who may have been a one trick pony.
We also have the thought that he can help the defense to prepare as a scout team QB in terms of the read option. If anything, it never hurts to practice tackling a 6'3" 240 lb football player--no matter what position you'd have him playing. It's possible that Tebow may be able to take some short yardage snaps at the position of RB in goal line/2 pt conversion situations.
We know that a player of that size has to offer something as a blocker, so if Belichick is able to coach him up a bit he'll most likely put his heart and soul into at least contributing as a blocker in certain situations (I'm assuming).
To sum it up--Mike Kafka obviously couldn't do any of those things, and unless he was somehow progressing as a downfield passer and we just don't know about it, I see the move to add another QB as a positive, because he's probably much more of an addition to the "QB3" position than adding another guy like Kafka may have been.
We know the kid is a good athlete when put in certain situations, and we know that he's proven to be a pretty efficient leader too judging by his experience not only at FL but taking a pretty shoddy DEN team and helping to lead them to a playoff berth and upset of a good team. I agree with ausbacker that he isn't a "great NFL talent," but are probably some things that Belichick sees fit to try and make the most out of too.
If anything we can just replace Brady with Tebow vs PIT and let him sling the ball all over the field for 350 yds