As far as I know, tangents are used in Sins for both bump effects AND lighting surfaces
You know very well 
if your tangents are missing or are badly made there are obvious lighting artifacts on the model. Which in my opinion is a much uglier problem than a seam here or there.
No tangent is not a really a problem... if tangent is set on FALSE in the .mesh, the GPU calculate them... and in case of modern graphic card, GPU make a good work... it become ugly when GPU don't support shader 2.0...
Well, there is a other problem... if there is no tangent, the convertxsi set tangent on 0,0,0 and U1/V1 on 0... problem is that GPU shader need to know the handedness ( orientation, http://en.wikipedia.org/wiki/Orientation_%28mathematics%29 ) for calculate a good lightning/shadow in sins... convention are that if tangent are 0,0,0 then U1 is 0 and V1 is "+" or "-" 1...
So, with no tangent, i need to edit manually the .mesh created by the convertxsi and set the V1 to 1 for each vertex... if during the modeling process, one triangle ( poly ) was flipped, V1 will be -1... once this done, lightning/shadow is pretty good in game... ask some resource from the GPU but with SOASE, the GPU is never overloaded like RAM or CPU...
As for seam problem, it is maybe the wrong word... what happen is that a piece with bump map have some detail who bump out on a piece and who are like hole on the mirrowed piece... again a problem of handedness... not only the seam will look bad but all the surface of the mirrowed piece...
And yeah, you can avoid symmetry problems by hiding or avoiding seams with geometry. Not always possible , since that technique depends largely on the model design.
It is not hiding but it is avoiding... and it is always possible... it is mainly related to the way used for make the model... a single object model cannot use the trick but a multiple object model can... the model that i work on now is made of several thousand little object... primary reason is some poly count optimization... by example, the barrel from the gun show below are made of 4 object... it allow me to use 56 tri in place of 68 tri is the barrel was made in one piece... 12 tri spared don't seem a lot but since the model have 122 turret with 2 barrel, i spare 2928 triangle on the full model... and in the same time, it spare me the seam problem...
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As for shadow and light without tangent... first picture below is with the .mesh generated by convertxsi, where tangent is set on FALSE, tangent is 0,0,0 for each vertex and U1/V1 are set on 0... shadow/light are calculated at the poly level, jumping from one poly to the other... something ugly to see... second picture is with a manually edit .mesh where i have replace the value of V1 by +1 ( right handedness )... shadow/light are soft ( calculated at pixel level ) and move smoothly from one poly to the other... so, the no tangent problem is mainly due to convertxsi setting V1 on 0 when there is no tangent... correct it and result is greatly improved...
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Of course, pre-calculated tangent in XSI are good but don't work good in my case... first, the neutron star is light emitting via particle but since it is not part of the main model, it give strange effect on the illuminate part of the model... same problem with the rotating ring who are a particle effect too... so, calculate the tangent/light/shadow/bump in a dynamic way is somehow the only good solution in my case... and only the GPU can make this job...