This is really interesting because of the differences between eukaryotic DNA segregation and plasmid segregation. Plasmids a essentially circular pieces of DNA the attach at both ends. The plasmid is generally condensed, very similar to how a rubber band twists up. The potential energy stored in the condensed plasmid configuration can be used to propagate certain interactions. Since our cells utilize microtubules in chromosomal segregation, I wonder how microtubules interact with plasmids; their structure is very different, so I imagine their segregation is similarly different when compared to DNA.
The amino acid chemistry of Beta-tubulin must make it more conducive to the propagation of microtubule growth. Since it is a GTPase, perhaps it has a higher affinity for GTP and consequently more likely to bind to another tubulin protofilament than the alpha or minus end. It's interesting because there is still addition to the microtubule from the minus end, just not as much as the plus end.
I wonder what benefit the alpha-beta seam found in many microtubules does. If it is a regularly occurring molecular orientation utilized by cells, then there must be a purpose for it. Perhaps it enhances the flexibility of the microtubule, either positively or negatively, as a way to regulate its function. For example, sometimes microtubules need to be more rigid than normal, and sometimes the opposite is true.