Meh.
You've got $1 dollars(20th pick in the draft) in your pocket and you go to a store with the intentions of grabbing a box of cereal prior to a hurricane. Problem is you're competing with other people (teams) doing the same thing. You arrive a late (Outside the top 15 picks) and most of the quality items are gone. For example, instead of Kellog's cereal(top 15 talent) there's only the great value cereal left. But then you see it...a box of Frosted Flakes(Top 13 talent)...sitting way in the back. Problem is, the reason it's there is the box is open, and the bag has a little tear in it (slight injury history, or questionable character concerns).
You take it to the front, and while the cereal is regularly priced at $1.25, the owner is willing to give it to you for 75 cents. If you really have a preference for that cereal, you likely take the discount and walk out the store feeling lucky. Yes, there's a high chance it's stale...but there's a chance it's not and because you started the night at a disadvantage (fewer picks than other teams, or drafting late) you saw a potential bargain that you otherwise wouldn't have had a shot at, and you take it.
The Saints, since 2006 have had a top 15 pick only 6 times, not counting trade ups. They've picked in the top 5 only once in that time span (2006) not counting Sed Ellis.
It makes 100% logical sense why they take chances on players that otherwise they wouldn't have access to...because if you hit (Ryan Ramczk, Bryan Bresee etc) you hit big and if you miss, you got him at a discount anyway. Sure, they could take the great value cereal and end up with a steady great value player level player, but this is a talent driven business, so they go after the more talented players despite them having some issues.
It logically makes sense from their standpoint and as long as it makes sense, the decisions can be justified. It's when they do things that don't make sense that are questionable, but passing on players you aren't interested in because they don't fit what you're looking for is completely plausible.
I digress