This is partly for visuals, as both current casting blocks would make items inside difficult to see when upsidedown. This is partly for gameplay, as there will just be fewer gas casting recipes. In addition, slimestone may be used for some new channel variants to allow pumping fluid upwards...
#TINKERS' CONSTRUCT IS UNDERGOING A REWRITE The current 1.7.10 version of Tinkers' Construct is the final one for this iteration. Only major bugs will be fixed. The rework is aimed at 1.8 and has no time-plan. It'll be ready when it's done. ...