I'd suggest the Federation went to the Roms for a cloaking device as their cloaking technology should technically be more advanced than the Klingon variety. In addition, for a Fed ship to use a cloak would be a contravention of whatever agreement they'd signed with the Roms (the one they mentioned in TNG episode with the Pegasus (was it the Pegasus, I've not seen it for so long I've forgotten)), or risk a political scandal.
As for the Defiant having to decloak to use the transporter when the ST:IV BOP did not, there could be many reasons - demand on power, for example - but most likely it was just a typical case of sloppy writing and dis-continuity.