Washington was signed for JAG pay. The team is paying $80K in bonuses to have another player competing to make our roster at WR, competing with the likes of Martin, Dobson, Boyce, Harper and Carter. For me, there is no downside. If he moves up and deserves more reps, then that's great.
The team has added our 3rd WR. When Edelman is healthy, we will have Edelman, Amendola and Hogan. Maybe one of the rest will be a major contributor. Personally, I wouldn't expect ANY of them to see the field more than a couple of times a game, unless there are health issues. IMHO, when we run a 4 WR set (rarely), I'd expect to see Lewis on the field, rather than any of our backup WR's.
SIDE THOUGHTS
I see a major hole is Amendola is not on the team. Also, I expect us to draft a WR early , although this seems to be a poster's dream every year. In the end, I can't argue with Belichick's believing that he has a better shot finding a WR in free agency. Hogan and Washington may be our WR additions for the season (along with another JAG and a late draftee).