Since Portable Tanks have well defined input/output faces (i.e. up and down), mechanical pipes should avoid connecting to the sides of a Portable Tank.

Not gamebreaking at all.

In prerelease the tanks originally worked that way - only allowing connections on the top and bottom. However, I later realized that the tank could also be used as a Mekanism-based Fluid Transposer (filling/emptying buckets), and that would require both a fluid input and two transporters being connected to the top and bottom in order to import/export fluid containers. BuildCraft tanks also allow for pipes to connect to their sides and modify their contents.