Yes I agree entirely with your post 
I think a classic example is someone saying to themselves "I'm probably going to PvP in warburg and RV, and I'll do some exemping to help friends (mostly high level)"
In that case I'd personally go for level 41 set IOs (if the build could benefit from set bonuses significantly) and take the small hit in performance at 50.
And your last point about chaging to the level 50 MB IO is a very good one, I never even thought about that, but it seems so obvious now!
i think this thread comes down to: SO's are good *thumbs up*
but IO's make you better *bigger thumbs up*
Also if you can see yourself playing your 50 alot then sure why not get io's at level 50, but because io's dont loose their effectiveness you can use some of the lower level sets (with higher than average bonuses) effectively too to reach the ED cap still. AND get the bonuses available at them levels.
Oh yeah, and there are bonuses at lower levels that are more desirable than some of the others at high levels, and will stick with similar bonus enh at higher levels. Best combinations would be between the higher tier enh and lower tier enh.
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). But in that example the power could still be improved by changing the Mako's Bite 4-way IO to level 50, since it won't affect set bonuses with only one Makos Bite IO slotted! 
So you can see that if I stack up my set bonuses I'm going to start seeing advantages.
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Certainly... but you'd need to make the tradeoff of functionality-at-30 for more overall enhancement when you're not working at 30.
For toons that know they're going to be doing most or all of their play at a particular level it makes sense to build around what's going to work at that level.
The perfect example would be a level 50 toon who knows they're only going to do Sirens PvP... They would likely work out their possible set bonuses at 30 (+damage, +recharge, +accuracy or whatever) and measure them up against the possibilities opened up by a mixture of level 50 HOs and IOs.
For *my* toons I generally know that I'm likely to be doing TFs at many different levels, or a mixture of Zone (25, 30, 38, 50) and Arena (50) PvP. It makes sense to me to focus more on the maximum enhancement values (since they always apply no matter what level I'm working at) and work out my set bonuses for level 50 rather than trade some of those enhancement % for lower level set bonuses which I will only get the extra benefit from when exemping to a particular level range.
Obviously milages vary, and I see your point on the Sirens PvP example (though I'd likely opt for +damage set bonuses over +accuracy, and stick a low-level 'Global 6% Tohit' Defence IO in somewhere!