The difference between a 3rd and 4th is actually pretty significant: A player picked in the 3rd round has about a 25% (or 1 in 4) chance of turning into an NHL player. The odds in the 4th round drop to about ~15% and then down to ~12% for rounds 5-7. In other words, after the top 90 or 100 players are picked, there's a significant drop-off in talent. That's why a 3rd would have been nice. Mind you, it would have also been a pleasant surprise. I don't think expecting a 3rd for a player like Soshnikov was realistic.
Ultimately, I think this is a good buy for St. Louis. Soshnikov has the toolbox to be a quality 2nd or 3rd liner in this league: The skating, the shot, the willingness to engage physically is all there. The only question marks are his hockey IQ and whether his body can withstand the rigors of an NHL season. So far, the answer has been no. At worst, they've got a decent 4th liner who can kill penalties. All for a pick unlikely to turn into anything.
From the Leafs perspective, Lou and co. got what they could get, but I feel as if they sort of backed themselves into this corner. They built up quite the stable of young, NHL ready forwards but have a coach who has a strong preference for the veteran and refused to move on from players like Leo Komarov. Not what I'd consider to be strong asset management. Fortunately... they also seem to be good at finding these guys. Sosh, of course, was a free wallet in the first place. Here's hoping the contract spot gives us some flexibility moving forward and that pick actually gets used on someone talented, not another hulking defenseman with questionable skating.