How about a slightly different calculation ?
The Pats keep 3 QBs including Tebow and they use him as needed for each role. Then, they don't need to keep an additional TE, an additional RB, an additional WR and add a FB ?
That frees up quite a few roster spots. The slots in question are all bottom of the depth chart for their position, so it's not like your getting "prime" talent in those positions to begin with. Rather, you will be getting someone "serviceable".
Don't get me wrong here, I'm not saying that they could or should try to turn Tebow into TE1 or TE2. But he could serve as TE 3,4, or 5, or WR 7 in a slash type role, or a RB3 or FB2 on a fill in basis, if only to spell others at the position and give them a breather.
It's a case where he might not (and probably wouldn't) be better than any one individual that he would be replacing, BUT, his versatility could more than make up for it as it frees up roster slots.
(BTW, I should add that I'm personally against this as I'd prefer that he just be a QB3 and spend time developing at that position. But Bill and company aren't knocking on my door asking for my input, so it's a moot point
)