The Tur writes (Orah Haim 11) that since in his days they didn't have Techelet they didn't have to be Makpid on the Huliot (the Rambam doesn't agree). Therefor the Posekim hold (Rosh, Tur, SH"A etc.) that we don't do 7-13 Huliot because it only is for Techelet. However, in the time when Techelet exists then you should tie the Sisiot with Huliot.
A few questions regarding this:
1) What does the Techelet have to do with whether we tie the Sisiot with Huliot or not?
2) If we wear Techelet MiDin Safek Deorayta (like we do nowadays) must we tie with Huliot or can we keep our old tradition (i.e. 7-8-11-13)? It seems Rav Mois Navon answered this on Tekhelet.com but he didn't say why we should be able to. It seems that we actually should have to switch over to the old Minhag of the Rambam (or Rav Herschel Shachter and the like) to tie with real Huliot.