I don't agree with this, personally. I don't think you can have enough depth, or competition. Our ST is probably the best in the league, so I don't think we have exactly cannibalized STs to get roster depth.
I think that you draft to improve the team, and for the long term. Trying to think in terms of available roster spots just doesn't work out well. There are invariably unforeseen circumstances. We went from having the strongest and deepest TE group in the league to being thin overnight in 2013.
Versatility is key. I don't prioritize a LB high, but someone like Bernardrick McKinney who can play ILB, OLB and sub rusher in different schemes is a highly rated prospect in my book. Having a rotation of pass rushers and keeping guys fresh was key for Seattle last year, and loss of players led to a major problem earlier this year.
The draft is a crapshoot, so trading away picks means putting less eggs in a smaller basket. Less chance for success. I would trade up if I were fairly certain, but a 1st and a 2nd is a lot. I thought Aaron Donald was worth that last year. Right now I don't have anyone I value quite that highly, but it's early.
Trading into 2016 is an entirely different consideration, but it depends on someone being willing to dance. I'd be thrilled to have a shot next year at someone like Jalen Ramsey, who I think would be a perfect fit for our defense.
I always enjoy reading your post Mayo and though I did mention a post of your in my above post do not think I was trying to put words in your mouth saying you do not support my position (though as you said you don't generally agree... but I understand this is a conversation of shades and not black and white).
I you mentioned the DL which tends to rotate a lot. I am fine without having many if any STs in that spot as that is the one spot on D you really need a rotation on and generally those players are too big and slow to do a lot of STs.
My comment is more for DBs and LBs and skill position O spots.
You mentioned the Patriots having the best overall special teams in the league and i agree that it might be. That is in part due to guys like Wilson, Harmon, Ebner, Bolden, Slater, Tyms ect...
Though you have rare example like Edelman that is a good WR and good ST player trying to get a bunch of guys like to put together your unit is not very likely to happen.
So would you prefer to sub the STs specialist out for better depth for O and D? There is an argument for doing such of course. However generally (besides DL as you mentioned and i agree on) you don't tend to rotate starters for the sake of keeping them fresh.
Generally you play your best players cause they are why you win. Though there is value in having a backup to McCourty (and i think we should have a better one) I think to try to get a 4th or 5th safety back up is being over redundant at the expense of hurting a need.
Here is how i see the roster (generally... not in all cases) for what you want to build on D.
DL 9 players.. no STs
LB 6 players overall including 2 STs specialist
DB 10 players overall including 2 STs specialist and 1 player who can do both decently.
I think this is the best way to build a team and BB tends to agree for the most part.