ideal spreader attachment

[Brion Toss]

Let's see, first a rigid connection is not necessarily a bad thing, assuming that the lead of the shroud is fair over the tip of the spreader; you don't want that shroud to be pulling that spreader tip significantly fore or aft. This can be an issue with bendy masts, but even here the spreader angle can be set for the fairest leads when the loads are highest.
Many spreaders that have "some allowance for the spreader to move" are just originally-rigid setups that have become deformed from unfair loading. There are spreaders intentionally set up to swing, but these have their own issues, especially the tendency of the spreaders to slam around on the leeward side, imposing very high fatigue loads on the components, including the mast. It can be done well, but it must be done carefully, and for most boats it just isn't worth the effort.
Welding on aluminum spars is not straightforward. It is a tricky job, requiring a high degree of skill and knowledge. Many, many rig failures are directly attributable to weld failure. Re the assembly in the picture, I have no way, from here, of knowing if the right stick was used, that penetration was complete, that surfaces didn't deform, etc. The thickness of the weld looks quite, um generous. In general I prefer no welds on a mast, partly because it is so difficult to find someone who does it well, and partly because there are usually better approaches.
For instance, the through-bar, with spreaders socketed over, its ends, is strong and simple.
Compression from the spreaders is usually not an issue, given the relatively low magnitude relative to bearing surface.
Fair leads,
Brion Toss

[brm3]

Thanks all for your input. I'll just chew on this a while. It sounds to me if I can keep my stay angle bisect properly and keep the spreader from walking down I should be ok.

[Robbie.g]

Just to be clear the spreader assembly in the picture has the welding on to a separate band.
You quite correct that welding directly onto a spar should be done carefully and with the correct materials and techniques, as the heat applied in the welding process can adversely affect the performance of the spar.
The need for a solid spreader attachment is much more important 'now days' as many rigs have a designed degree of sweep in the spreaders, and therefore the load path is somewhat less straight forward [for want of a better term] than a more 'traditional' inline spreader rig.
More of a horses for courses kind of thing, i guess.
Good discussion.
Regards