How necessary is the sub

Not necessary per se, but it adds a significant amount to the sound.

It depends on the kind of movies you will see. The classic dialogue heavy ones aren't the ones that need the Sub. But if you are into seeing the effects laden superhero/war/action/thriller kind of movies, the Sub is where most of the effects come from.

Like others have said I found the soundbar alone was great but couldn't resist a sub once I had a demo of this and adds significant oomp. I found that also when playing music this helped others speakers in the same zone as bass was offloaded to the sub and meant they could concentrate on providing a great sound 😃

But if I were watching something like Casablanca
I would enjoy Casablance just as much on an old CRT TV with its built in speakers!
On the other hand, scenes like the lift off in Apollo 13 can use all the sound enhancements one can afford to provide it.

I have a playbase and pair of 1's, I was wondering if its really true that adding a sub would increase the quality/clarity of sound produced by the mid-range's and woofer in the playbase(this is currently my setup, I am assuming same is the case for any other reconfigured zone).

Although theoretically it sounds plausible because,
Sub's possible freq range 20–200 Hz.
Woofer's possible freq range is 40 Hz up to 500 Hz.
Mid-range speakers possible freq range is 250 to 2000 Hz.

Now if a signal contains low frequencies as well as woofer/mid-range, I am assuming the sonos drivers would ask the woofers/mid-range speakers to deliver both of those frequencies and quality of one of them would be lost. But adding the sub would enable the woofer/midrange in the playbase to play their dedicated frequencies.

Since I dont own a sub I cant perform a test of any noticeable improvement in quality/clarity of sound produced by non-sub speakers because a sub was added to a zone.

I see similar signs from other reviews
https://www.amazon.com/gp/customer-reviews/R2038TF77LX95V/ref=cm_cr_arp_d_rvw_ttl?ie=UTF8&ASIN=B01HL8LXQ8
as well as sonos community member

This is true. Anything below the limits of the Playbase speakers is rolled off, but they are still tasked to play bass that would be better served by the Sub. Any power used to play those bass frequencies would be freed up for use elsewhere if those frequencies are diverted to the Sub.

Any power used to play those bass frequencies would be freed up for use elsewhere if those frequencies are diverted to the Sub.
A question here, for Sonos to supply an authoritative answer.
Where a Sub is bonded to a play unit pair for music, low frequencies hitherto sitting in the same 2 channels are diverted to the Sub leaving less for the play units to deal with, allowing them to then deal with those in an improved manner.

Does this apply for the 5.1 effects as well? Since surround and the .1 effects are contained in distinct channels, does the play bar/base play these as well when the surround speakers/Sub are absent? If yes, the same improvement will be present in the bar/base when the latter are added. If no, the surround and effects content will be additional sound presence and a very noticeable improvement, but with no effect on the bar/base sound quality per se. Of course, given the overall improvement perceived, this is probably of just academic interest.

More important is the vice versa situation - when the Sub is absent, do the effects contained in the .1 channel get played by the bar/base or are they absent as well? Likewise, the question arises for the surround content too.

tl:dr Life was great before it arrived, but better after. Not required, but adds the "Wow" factor.This was exactly my experience. I was happy with the Playbar, but it lacked that impressive edge the Sub brought. I wouldn't have had a problem before buying it, but I couldn't go back now! :S

That's a typical outcome with the Sonos Sub. Those getting it on a test basis are therefore well advised to do so only if they have the budget to buy it if they like it.