The 4th/5th receivers tend to not as good as the guys ahead of them so they wind up being the 4th/5th receivers.
Few teams have Megatron type talent as the last guy on the depth chart.
Yep.
Once you get past the 4th WR on a depth chart (of any team), your value is pretty much Special Teams and nothing else. You look around the league and take a look at any teams depth chart, and then look at the bottom 3 WR's on that team. I guarantee you those 3 WR's are either rookies, project players, or players who only contribute on ST's. VERY few teams get anything out of their bottom 3 WR's (outside of ST units).
Now, look at the Patriots. Due to our TE's we essentially only have THREE WR's that matter. Welker, Lloyd, Gaffney. After that...it's just project players and ST'ers. And that is where guys like Edelman and Slater shine. Sure, they are the two bottom WR's on our depth chart, but they are also ST standouts, and that is the sole reason they are on the depth chart in the first place.
The Patriots may not have a "Megatron" at the top of their Depth Chart, but as a whole, very few teams can claim to have two ST aces like Slater/Edelman, two consistant clutch players like Welker/Gaffney, three proven Veterans in Lloyd/Stallworth/Branch, and....well...OchoCinco.
We may not have a Calvin Johnson, but as a whole...as a collective group...our WR depth is pretty darn good, IMO.