What would preposition "sewi" mean? "poka" exists because we want to have "with", one of the more basic prepositions present in almost all languages (if not all), it was chosen to reduce word count, since it would have been "kan". There is no preposition that could be "sewi", unless you're refering to "up", but akin to "up" is "next to" which is "lon poka", not "with". That's not a valid attack on "poka" as preposition.
As for "mi tawa poka sina." It's easier to say "I go to your side" as "mi tawa sina." and probably "mi tawa poka sina." ("I go with you.") might also be expressed as "mi tawa lon poka sina." ("I go next to you.", see how they seem equal even in English), although longer. That's not to say "poka" equals to "lon poka", just that in some contexts, it really doesn't seem to make much difference. But in many other contexts, it'd be strange to say "next to" instead of "with" (I build a house with you/in your company. vs. I build a house next to you/at your side/near you.). Since that seems to give you zero bothering, then you have your argument justified. nasin pona li mute.