I wouldn't call any major manufacturer's adaption of a bolt action to muzzle loading a gimmicking. Rather, they're trying to gain a market share of a growing market.
BTW, was told at Remington that they use the same steel for their barrels on the muzzleloaders as they do on the regular cartridge guns.
Turning to the question, the advantage of the bolt action in-line is quicker lock time. However, even if one was offered cheap, I still perfer the older Plains style or Long Rifle (or even military musket) look. Curmudgeon me.