ringo is actually a decent singer and not a bad songwriter. but being a drummer and singer, especially live, is very strenuous cause you have to keep the beat, whereas guitarists can think of the tune cause more often then not the tune that they play is the same as the lyrics they're singing. drumming and singing takes some real talent, and he does it beautifully.
I see what your saying but have to dis-agree with this drum and guitar thing. The dilemma is the same for both guitarist and drummer.
Knowing the melody helps both musicians. In some ways at least a drummer can just keep time (Even though Ringo had his fair share of time changes)...A guitarist can have time, pitch, harmony, melody, effects, vocals to consider all at once. Same with bass playing and singing which also takes great talent what with all the counter rythms, melodies and lyrics to remember.
It just applies to any musician. I wouldn't say one is harder than the other...You get better at doing it with practice.
How you show it is a different matter...Ringo was definitely a great drummer, average (Although sincere) singer and an occasional songwriter with mates who could make them songs into something great.
He is a great chorus singer. Most of his Beatle tracks are great sing-a-longs. I always remember Ringo's voice the most out of all of them in Carry That Weight.