Filamentous prokaryotes, unicellular eukaryotes: Meh, so what.

Now I don't know about fliamentous prokaryotes, but according to Pharyngula the introduction of mitochondria allowed eukaryotes to have much larger genomes. As mitochondria contain nothing but the essentials to generate ATP, the cell has enough power to synthesize many different proteins and RNA strands, allowing a big and complex genome. This both increases the rate and searchable area of evolution, and makes it possible for multicellular eukaryotes to evolve. From Lane and Martin (2010):

The conversion from endosymbiont to mitochondrion provided a freely expandable surface area of internal bioenergetic membranes, serviced by thousands of tiny specialized genomes that permitted their host to evolve, explore and express massive numbers of new proteins in combinations and at levels energetically unattainable for its prokaryotic contemporaries. If evolution works like a tinkerer, evolution with mitochondria works like a corps of engineers.