In a 20+ year old copy of the TIA/EIA 568 spec it says:
"The cable shall consist of 22 AWG to 24 AWG thermoplastic insulated solid conductors that are formed
into four individually twisted-pairs and enclosed by a thermoplastic jacket."
High frequency travels on the outside of a cable. Much like wave guides. If i had a cca cable that had the ends on it i wouldnt care a great deal, but if you even scuff the coper around the aluminum it effects frequency propagation, not only does it suk just in general. Scuffs on a solid copper conductor can also effect high frequencies. At least thats what the 4 hr class was about 30 years ago...........and i was hung over.