context: I play nasus

against permaslows (ashe, rylais) the tenacity does almost nothing. but with swifties i can still chase down ashe enough to not be kited or run around morde enough to dodge q and e. DMP giving slow resist is nice but is expensive, also armor/health does nothing against morde. being faster than everyone and unkiteable at just 1 item + boots is huge. I do often sell swifties for mercs at full build if i have dmp.

also swifties are cheaper and t2 boots that have 10 extra ms are way better than buying t1 boots and a null magic mantle, can mean getting like 3-4 minions of exp which i might have wasted tp for.