Only needed on high power amps from a power handling standpoint. From the mechanical security, indestructability, and lack of exposed conductors, they are an excellent idea on ALL amps. Unless you live on the backside of the moon, speakon cables should be almost as easy to find as 1/4", albeit a little more expensive. If you make your own, though, you can go speakon for less than any store bought 1/4" cable of any quality . . . about $10 for 3-4 foot long, 4 wire, biamp capable . . . . and a buck or so less if you only go 2 wire.
Regarding the sound question, 1/4" sound fine . . . right until they fail, and they they sound like total crap . . . . been there, done that, more than once . . . unfortunately . . . .
- Tim