I'd love Marner but we don't have the draft picks to even offer sheet him smh. And if they did, this unwritten rule you don't offer sheet players is frustrating.
GMs don't do it for multiple reasons - only one of which is because of the "unwritten rule".
1) The compensation rules on the good players make it prohibitive.
2) If you're targeting a low level player who won't get much for compensation, you'll A) likely need to overpay that player and B) the other team will likely match it and deal with the cap issues later - unless of course you drastically overpaid the player. Then they'll just take the compensation and let you deal with the meh contract.
3) You're almost certainly ruining your cap structure by trying to overpay a player.
4) If the player is that good, the other team will just match and deal with the cap issues, so it's usually a waste of time.
5) It open's up your player's to be targeted - not really something you want to do if you can avoid it. Remember, payback is a bitch.
So while yes you can do it... the benefits vs the risks isn't all that high. You're either drastically overpaying for marginal players, or you're just pissing off other GMs as you try to inflate some player contracts that they're just going to match anyway.
I mean seriously... what RFA player in this year's FA list are you going to OS? Because as I go down the list of players, I can count the guys who would be realistic targets (vs dreams like Laine/Conner/etc) on one hand - missing digits.
I honestly think HF peps look at OS the wrong way. I don't think it's so much a matter of a tool to allow GMs to poach other team's players as it is a tool for the PLAYER to ensure that they're getting a "fair" market rate. It prevents a team like Nashville from lowballing the crap out of Weber. It's not so Pittsburgh (or whomever) can try to pilfer talent from a cap strapped team. And that's exactly how HF wants GMs to use it.