No, it's not really necessary. But thanks to me posting pictures of my silly new FS bike, everyone and their cousin wants a curved seat tube. And so I spent a couple of hours this morning cursing a Supertherm 1/7/1x31.8 "seat" tube. Yes, you can curve butted tubes - but only if the center section is thick enough, which this one is. And it needed to be pretty beefy, this frame is a bit of a monster.
Exhibit 1: The downtube is a 44.5mm Supertherm job, so big that it swallows the seat tube miter whole! After trimming the ears down, it just barely can be welded in without looking weird.
The toptube, too, is Supertherm, and the head tube is a big 44mm ID for a tapered steerer fork. All this big beefy tubing almost makes the Deda 30x16mm/1mm wall s-bend chainstays look wimpy (they're not - not at all). The plan is to use some beefcake seatstays too, with rack mounts for adventure touring/Great Divide/etc.
Should be a fun all-around adventure bike. Here's the relevant geometry for curious people:
-70 degree HTA with a 128mm head tube. Built for 100mm travel tapered steerer fork.
-75 actual/73 effective seat tube. 49.5cm/19.5" curved. Neato.
-Clearance for 2.5+" tires.
-44cm chainstays (at the forward-most adjustment of the sliders), 315mm (12.4") bb height.
-Supertherm tubes throughout, way beefy. For a 12x135 Maxle (Paragon sliders).
-S-bends, direct mount front derailleur (not on there yet!), rack mounts, 3 H20 cages, ready to rock for carrying a big load far from civilization.