I feel you 100% on this!
So many teams with expensive/high Live Series diamonds and they give you a diamond that if it were on the market, would barely be above quick-sell value -- i.e. cards like Garret Anderson (Angels), Robin Roberts (Phillies), and Shane Victorino (Red Sox).
Out of all the cards you get for collecting the Live Series (I only need deGrom, Arenado, and Freeman it), the only cards I found remotely useful so far have been:
Jason Giambi, 92 OVR (Yankees collection)
This card rakes so hard. I was so excited to get the Roberto Alomar, but as it turns out... Giambi was the real gem. I had a game last night where I hit 2 HRs and a double with him and that felt like an average game for him. I don't know what it is with this card but everything just jumps off his bat. The only knock I have on this card is Giambi's lack of speed (29). I didn't think I would be replacing the FotF Matt Olson until a Prime/SS Frank Thomas was released but Giambi can even mash lefties, something I was a bit spotty doing with Olson.
Cody Allen, 88 OVR (Indians collection)
He's in my bullpen for now, but I can already tell it won't be long until I'll find someone to replace him. I like him a bit better than the Robb Nen but I may give Eric Gagne a try after I finish off the Live Series collection (only need deGrom, Freeman, and Arenado!). I wish he threw a little harder, I'd probably feel a bit better using him. But he'll be replaced as soon as I get Arenado back and lock him in to get that Adam Ottavino.
Sean Doolittle, 88 OVR (Athletics collection)
This card -- on the other hand -- is excellent! He's easy to control, has decent break, and a silky smooth delivery. The only problem is, I feel like I have too many left-handed relievers I like to use with Billy Wagner, Aroldis Chapman, Amir Garrett, Josh Hader, and now I also have this Dan Plesac card. I sold off Zack Britton because of this very problem, but even so, I still really like this Doolittle.
Vladimir Guerrero, 91 OVR (Nationals collection)
This was the first collection I targeted to get done, and while that was a good choice in hindsight considering I paid maybe 65K for Max Scherzer (he's now around 120-130K, this Vlad didn't stay on my team very long. I loved his arm strength and he hit a ton of doubles and got on base a lot, but the power just didn't play up for me. I hope we get a better Vlad soon as he's one of my favorite players of all-time and this was the first year I got to use a diamond card of his.
Mark Prior, 86 OVR (Cubs collection)
This is a very underrated card. I think this was the second or third collection I finished so Prior got some run in my rotation before I upgraded on him. He has a nasty slurve, which is always one of my favorite pitches, and was a strikeout machine for me. I don't know what it is about Cubs diamond pitchers, but I always do well with them (Jake Arrieta the last few years and Fergie Jenkins last year).
Mike Piazza, 92 OVR (Dodgers collection)
I just bought Bellinger and unlocked Piazza about an hour ago, and while I have yet to play with either of them, I am so excited to do so! Bellinger was easily my favorite card last year, I finished with something like 172 HRs with his Live Series card before I retired him. Piazza was also a card I really liked using. He was my end-game catcher (well, him and Finest Yasmani Grandal) and this version looks pretty good too. I unlocked Craig Biggio earlier this week and had been using him at catcher but I may move him to 2B or left field now and let Piazza take over for awhile.
Other collection rewards I haven't used yet: Adam Ottavino (Rockies), Dan Plesac (Brewers), John Olerud (Mets), Ryan Klesko (Braves), Steve Finley (Padres), Luis Gonzalez (Dbacks), Walter Johnson (AL Central collection), Willie Stargell (NL Central), and Mike Schmidt (NL Eeast)
There are a few of the golds and lower diamonds I do like such as the White Sox Mark Buehrle (82 OVR), Rays Evan Longoria (85 OVR), Mariners Robinson Cano, Reds George Foster (85 OVR), and Cardinals Jason Istringhausen (85 OVR) but I have to say I feel pretty underwhelmed by cards like the AL East Roberto Alomar, NL East Mike Schmidt, AL Central Walter Johnson but I can safely say that the rewards of Mickey Mantle, Craig Biggio, Gary Sheffield, Billy Wagner, Willie Stargell, and the other cards I mentioned make it all worrth it.