From experience, it can be an absolute nightmare for the landlord.
Instead of one single tenancy you now have 3 or 4, and if someone stays for 6 months, they have automatic rights to stay there for 4 years.
You'll have people coming and going from the house every 3 to 4 months, and will have to ensure that the accomodation is in rentable condition for each new tenant.
If one tenant starts causing hassle for the others in the house, but it falls short of anti-social behaviour, you are stuck with them for 4 years.
A tenant of ours went through 3 "sets" of other tenants - The tenant being older felt entitled to be "in charge" of the house and was obnoxious/ caused problems for other tenants so that they left in tears.
Also how would you deal with questions from prospective tenants asking what the existing tenants are like - do you lie or do you say "they have 'strong personalities'" etc.
Also some tenants will say "I'm not paying for the electricity for those two weeks in June - I was on holiday", "I don't use the tumble dryer - she should pay more" and "He uses more electricity than me" etc.
Furthermore, once you split the tenancy, it is very difficult to undo it - unless you sell/ do extensive renovations / need it for a family member.
DO NOT DO IT.
At best, allow the sitting tenant the opportunity to sub-let - let him/her have the trouble of looking for new tenants and you are protected.
BE VERY CLEAR TO THE NEW TENANT THAT (S)HE IS SUB-LETTING AND NOT LETTING.