First, let's define what is a "working cell phone":
It is a cell phone that can attach to a local public GSM/LTE network and 
that can call and receive voice calls from each phone on public networks;
otherwise there are many people that rely on using wifi access, that 
however is not available in the majority of places, so they cannot be 
called or call if they are out of coverage.
Also I ask people to have a reliable and fair connection; this means that
I ask guests to have a SIM either from their country of residence, or from
certain european countries. I say "certain" since there are many operators
that sell "tourist SIM", that in front of good or even free outgoing fares, 
charge an hefty surcharge [to caller!] on incoming calls, or once credit is
depleted they do not allow even incoming calls. 
For this reason, unless the sim is a normal one from the country of your
residence, I will accept temporary sims only from 
at, cz, de, dk, ei, fr, fi, hu, it, lu, no, pt, ro, se, si, sk.
This explained I have to explain why I am quite strict about my guests 
having a working phone that can be called even when they are not sneaking 
into a wifi network: 
It is italian custom more or less followed (but here in Firenze it is
extremely respected, up to have it written in municipal regulation, in a
stricter version than in the general road code!) that sidewalk are only for 
walking, and it is extremely rude to wait someone in front of the entrance of
the house. 
In the building Where I live, there are other 6 apartments where live also 
other 15 people that have the same entrance on the road so, if you were 
waiting there, you would be offensive to 16 people.
I never had problem with italian guests that know this custom, so they know
where to wait without being offensive to anyone, but even after telling
everyone that they cannot drop any luggage on the sidewalk and if they want
to wait they must do it on the benches on the other side of the road (that 
is also the most confortable place to do it) too many other people forget 
that rule and waited sitting on the sidewalk in front of the house,
surrounded by their non-levitating baggages.

It is anooying and I got often complaints by neighbors for this.
So I have to give my guests my address only when I am 100% sure that in case
i could not be at my home at the arranged time I can contact them, normally
giving them a part of address close to my home, where they can wait without
problem and calling them when I am home so when they arrive i can open the
door.

Incidentally having a phone avoid another possible problem when there are
more than one group: you can postpone dinner if someone is late, without
having the doubt if they were just late because they got the wrong bus or
had misunderstood times, or they was completely lost and could not arrive 
at a reasonable time, knowing all immediately.

Last but not least: if the guest do not have a phone I cannot even do any
kind of work either in the balcony or in the attic or in the garage, not
even in my bedroom, since in these palces i would not hear the doorbell, 
while a phone-bearing guest could alway ring me to the phone I have in 
my pocket and I can immediately go to the door. 
(once I had someone that remained out until 7 in the morning
because they went out for a post-dinner walk without telling anyone about it ...
and when they come back and rang no one heared the doorbell)  

Is saving just a few cents in roaming charges a good reason to inconvenience
a lot of people?

(I can make some exceptions, but only for people that arrive reasonably early
in the day and accept a confirm about time and place of meet only the same
morning - and I choose the time and place)