They had two options, as far as I can tell. Trade one of their top 2 prospects for a Lucroy, or deal a solid prospect and roll the dice that a guy like Bruce will give this offense a much-needed jolt. They went with the latter so as not to further deplete the farm system. I know plenty of Met fans who like Hererra, but clearly Sandy was not as high on him as others.
Bruce is a flawed player but gives them a big bat to go with Cespedes. Sandy has always liked him; tried to get him instead of Cespedes last year. I'm ok with it because, best case scenario, the trio of Thor/Degrom/Matz continue to pitch well (or even pitch better) and Bruce helps the seemingly always slumping offense. We caught lightning in a bottle last year with Cespedes - not only did he play like the best player in the game for a 6 week stretch, but others seemed to catch fire at the same time for whatever reason. Maybe Bruce can spark the same? Yes, the OF defense will be very suspect with Grandy in Center (because Cespedes refuses to play there), but they had to do something, and even though folks disagree on how valuable Bruce is (great HR/RBI totals, but poor defense and not a great on-base guy), he is a major upgrade for the lineup. If he catches fire and Cespedes gets healthy, we can possibly go on a run with those 2 driving the lineup AND some real good pitching from our Big 3 (plus Colon who is solid and potential upside with Wheeler).
On a different note, while I'm not a Yankee fan, I think Cashman did a hell of a job getting the level of prospects they did for 2 relievers (one who is a FA to be) and Beltran (also a FA to be). Their farm system is probably now ranked in the top 2/3 of baseball. The Chapman acquisition and subsequent trade was particularly shrewd. When viewed in the context of the Adam Warren for Castro deal, they basically got a few months of Chapman, a young, solid 2b (Castro), a solid, versatile bullpen guy who has done well there (Warren), 2 top 30 prospects (Torres/Frazier), another top 50-75 prospect (Sheffield), another top 100 prospect who was the #5 overall pick last year (Tate), another guy who is probably a top 100 prospect (Mckinnon) and a bunch of other solid level prospects FOR 2 months of Chapman, a year and 2 months of Miller, 2 months of Beltran, and a few other second tier prospects (originally dealt for Chapman in the deal with Cincy). That is a tremendous haul.