I must admit I had no idea of the depth of feeling this issue provoked. Personally, I'm mostly interested in the single-player campaign but I sometimes find the multi-player can be fun - it is certainly more fun if it links to the SP campaign. To learn that some people loathe MP so much that they threaten never to play the game at all is eye-opening. Certainly it is said that the completer-finisher player tends towards a certain...ahem...personality type. I see no evidence here to refute that.
Perhaps, if some are so easily offended, instead the SP campaign could link to MP, but not the other way around? Unlocking the alliance with the Dwarfs in the SP campaign, for instance, could unlock the Legionnaire in the MP campaign.
I'm a completionist, myself, and I also play MP. I don't think there's any specific tendency there, personality-wise.
But, yes, there is a lot of anger at the inclusion of MP. After a long effort, the forums at large do understand that the way the resources are budgeted fore them are separate, and that MP's existence doesn't, in general, impact the SP experience. But there is still a lingering grey zone, and people are very wary of anything that looks like it might cross it.
The MP element of ME 3 was definitely a success, flaws and all, and there was a huge community component between BioWare and the players, so there was little doubt that the process would be repeated as future games came out. But even so, the decision to keep the play experience separate was made to avoid provoking conflict with the player base, and between the players who play MP, and those that don't.
Since there's nothing preventing us from enjoying the MP experience, whether there are any direct links to SP, or not, I'm content that they are right to keep them separated.
As for why they wouldn't allow successes in SP to unlock MP items, that is largely a question of finances, I believe. The model essentially works for them because some players would rather spend money than additional time playing to unlock items. If they can unlock items for both modes simply by completing the SP campaign (something virtually all players are going to do) it reduces their motivation to purchase packs with real money (which is the revenue stream that justifies MP's separate budget in the first place.)