Can you honestly tell me that George Foster isn't an upgrade over what the Lions currently have at RT? No... you cannot.
As much as you may think that Foster is a "back-up linemen" (although he started 45 of a possible 48 games for the Broncos over the last three years... on an offensive line that is known to turn any player into a 1,000+ yard back)
Foster is a definate upgrade and will improve the unit GREATLY.
That gives the Lions a potential line of Backus-[---]-Raiola-Woody-Foster, which is a solid line... and much improved from last year, when the Lions' offense actually performed well (with names such as Frank Davis, Jon Scott, Stephen Peterman and Rick DeMulling *starting* games for the Lions).
This also changes their focus for the draft. Now that they've filled one hole, they can focus on improving other areas.
Now, onto Bell. With KJones out, Bell was the PERFECT pick-up for the Lions. They could've spent a draft pick on a young back (which would leave them two young, unproven backs), or gotten rid of one of the last "trash" pieces of the old Lions to improve two areas of need. Bell will thrive in Martz' offense... and will give the Lions a powerful 1-2 punch when Jones is healthy.
Bly. When he plays in a system that fits his talents, he is a playmaker. You MIGHT have forgotten his previous seasons... but he had great numbers and deserved his spots in those Pro Bowls. Paired with Bailey, they will form a great CB duo... you won't want to throw to one but will get picked throwing to the other.
You can pretend that this trade was simply a "convenicence", but you will be proven wrong. The Broncos secondary will be much improved with Bly and the Lions offense will be much improved with Bell/Foster.
i'm not going to say all three will be worthless players next year, but past history can't lead you to many other conclusions about bell and foster, and past history doesn't show me anything special in bly.