Mostly I'm talking about cannon here. CRIII cannon vs say CIII infantry in a fort results in more cannon surviving to thwack again later. Even better if the AI is running mainly mounted units. I haven't done much of this since barrage changed, but CR siege into forts is hands down better than any other promo for siege. Certainly though for pre-gunpowder fight vs a balanced SoD, CRII melee into a fort is the best you can get until after you pick off all the stack defenders.
Remember this is mainly a cheapass defensive trick against AIs. AI SoD tend to be predictable so you run a few suicide workers up next to them and get the fort up at whichever tile(s) they are reasonably sure to move toward. I like doing it mainly if I'm running HR (I tend to never research HBR so I can have cheap garrison troops until AdvF) as an alternative to flanking down the siege. I haven't run the numbers, but off the cuff I think this should be competitive in terms of
spent (particularly if you intend to go to war before your highly promoted surviving siege go obsolete) with flanking away the siege. Also this is an alternative to defending against the SoD before it pillages down some key resources (i.e. Marble or Fe)
It is a question of survivability question. Yes on a
per
basis I think the new barrage rules work out against CR fort trapping for siege (I honestly haven't spent enough time in the details here to know), but the more battles you have planned for that siege era, the more cost effective CR gets. For instance if you've just teched steel and shaka sends in a horde of muskets, maces, trebs, and knights your options to take down the SoD are to quick build some knights (if you have guilds) to flank away the trebs and then let your CG units kill the SoD; build some barrage cannons (drill cannons being useless here) and then mop up the SoD with muskets/maces, or fort trap and mop up in like manner. Fewer of the barrage cannon will survive, particularly once you hit a few more cities;
however CRIII cannon are like the Energizer Rabbit, they just keep going and going.