Cat 3 wasn't designed for voice either. It was designed for ISDN (channelized digital telephony), and Cat 3 was encouraged for new POTS installations so that those lines could be converted to ISDN later.
I don't lnow where you came up with that bit of trivia. CAT3 was the original ethernet after thin net (RG58 coax and BNC connectors) fell out of favor because UTP was easier to work with. There was a CAT4 after that. CAT3 was "encouraged" and still is for POTS because it is a twisted 4 pair cable and that makes it useful for premise wiring handling more than one line which was becoming more and more common at that time. If you put two lines on a 4 conductor J/K station wire that has parallel r/g/b/y conductors you will have a crosstalk problem. 4 conductor station wire was designed for one line and 10VAC from a plug-in transformer to light the dial light on Princess phones.
Multi-line business systems back then used 6, 12, 25 and multiples of 25 pair cable that was twisted pair but there was no CAT specification then. That only came about when data started using UTP cable.
There was never any argument for using Cat 3 instead of Cat 5 or 5e for POTS except for cost. Now that the cost is inverted, there's no reason at all.
The cost may be inverted at the usual sparkie or IT hang outs because CAT5 is all their customers know to ask for so that's all they stock. If you ask for CAT3 it's in limited supply or special order so it's going to cost more. Telecom supply sources will have normal prices. Also, if you look at the cost difference between CAT3 PLENUM and CAT5e or CAT6 PLENUM you will see that the difference is considerable.
Any of those telco forum guys who claim Cat 3 is "more proper" are full of it. Put them in the same box of luddites as anyone still wiring television with 300 ohm twin-lead, even if they're still using antennas.
Spoken like a true IT guy who never has seen or done a proper telecom installation. We all must be wrong, right? We don't use patch panels or 110 blocks. Our cross connect fields are 66 blocks and if you want to spend twice as much time untwisting pairs, not to mention the damage it does long term to your hands as well as the cost difference for plenum listed cable go right ahead. We aren't luddites, rather you IT guys need to get you heads out of your you know what and understand that the world doesn't revolve around you.
-Hal