I'd say Jagger's voice is a legitimate topic of discussion.
He has a very distinctive voice that is very well matched to the Stones sound in that it both helps define that sound but also the type of music they play and the way they play helps mitigate any deficiencies he might have as a singer.
I think this album choice by Benny is a well considered one, rather than just chucking a more obvious album at us. It showcases them trying a few different things and as part of that, imo, it highlights how Jagger's voice can be both a blessing and a curse in a way that listening to some of their earlier albums might not show. I think some of their attempts to do different things work well and others less so and I think Jagger's voice plays a part in that.
There are a some straight up and down rock n' roll songs such as When The Whip Comes Down where you'd struggle to imagine anyone but Jagger making a better job (though to Bimbo's point that song does include smart use of harmonies). But when it comes to the slightly off-piste choices it become less clear. Specifically:
Just My Imagination - I quite like their musical arrangement of this soul classic it has a really nice vibe and infuses a tiny bit of soul into the rawer rock sound very well; however for me it is an example of where Jagger can’t quite adapt his distinctive vocals sufficiently to properly get it over the line as a great remake.
More interesting still is Respectable. I don’t think I even realised at the time of release that Respectable was supposed to be a bit punky. You see loads of excitable 70 year olds online :-) explaining how this is the Stones handing punk bands of the time their arses and I think this is errant nonsense. To me it sounds like comfy rock royalty having a go at being a bit punky and failing miserably and part of this is Jagger's vocals. In theory Jagger has got a sufficiently raw and energetic voice to be decent at singing a punk song; however by this stage of their career his voice had become so iconic and well defined that it's almost impossible for him to step out of that cage it has created. It isn't clear to me why, if you wanted to sound a bit punk, you wouldn't put Keith Richards on vocals for those songs.
In fact the choice of vocalists is a bit odd/intriguing on a couple of songs. I understand that Before They Make Run is Richard's song but I'd be quite interested to hear a performance with his and Jagger's vocal roles reversed. The weird choice to try and become Dartford's answer to smokey robinson at the vocal bridge about 2 minutes into Beast of Burden doesn't really work for me either on what is otherwise an enjoyable track.
But then finally, or firstly, you have Miss You which is a diversion into some disco/funk influences where actually I think his voice works really well. At various point he dials it in and still manages to sound ok and the slight juxtaposition of Jagger singing something that doesn't sound like an archetypal Stones song works in it's favour rather than against. Similarly on Shattered a song that succeeds in sounding 'new wavey' in a way that Respectable fails to sound punky, I think Jagger does a good job too.
So a mixed bag for me vocally. But ultimately it's all down to personal preference and context too. Few people would suggest Ian Curtis's was technically as proficient a singer as Renata Tebaldi but that doesn't mean you'd want to hear her sing the line.. here are the young men, the weight on their shoulders
In summary I don't think Jagger is a particularly strong singer in the technical senses of the word and that shows in some limitations in his versatility but what this album does show is that his uniqueness for both better and occasionally worse is entirely integral to the Stones sound.
