This looks ridiculously lopsided now, but at the time that it happened the deal appeared fairly even. In fact, some pundits believed Hartford was getting the better end of the deal.
Ron Francis had been stripped of the Whalers' captaincy by Rick Ley and was having somewhat of a down year. Meanwhile, John Cullen was wracking up points with the Penguins, having scored 31 goals and 94 points 65 games when the trade was made. This wasn't the first time Cullen had surpassed the 30-goal or 90-point mark either as he had done so in 1989-90. Some felt the Penguins were making a big mistake parting with one of the league's top scorers. As it turned out, the change of scenery was just what Francis needed. Cullen, on the other hand, couldn't maintain the same scoring pace in Hartford. He put up a solid season in 1991-92 but it was nothing like what he had done in Pittsburgh and injuries started to hamper his career. Considering that he eventually ended up back in Pittsburgh by 1994-95, it goes without saying the Pens made the right move.
In the case of Ulf Samuelsson and Zarley Zalapski you're basically looking at a swap of a skilled, offensive defense man for a more physical stay at home defense man. Both guys were roughly of equal trade value at the time, but I think from a Pittsburgh perspective Zalapski became expendable when Larry Murphy was acquired from Minnesota in December 1990. The Penguins needed to beef up their blue line after having parted with Jim Johnson and Chris Dahlquist in that trade with the North Stars and Samuelsson was ideal.
As for Grant Jennings and Jeff Parker, well...the Penguins got 248 games (regular season and playoffs) out of Jennings while the Whalers only got 4 NHL games out of Parker...'nuff said.