The substitute sub-letter may not be compatible with the original group who may enjoy different lifestyles, tastes in music – which might irritate each other. Sooner or later someone will leave.
The important thing is that the original and departing tenant’s rent will be paid by the substitute! Should the substitute leave after a month due to incompatibility then the first departing tenant must once again pay at the landlords discretion under the joint and several liability rules of the tenancy. The substitute could also be called upon to pay. Any guarantor can also be called upon to pay up to the total amount lost. This is in contrast to a deed of assignment where the departing tenant leaves absolved of responsibility.
In light of this, it is arguably wrong that the original departing tenant(s) should be absolved from responsibility in the wake of such incompatible devastation? Not that there is any culpability in dropping out, but there is continuing consequential loss.
For this reason the original tenant will in some instances remain jointly responsible together with all the other original tenant-housemates until the tenancy expires. This can occur when the landlord agrees that tenants sublet to the substitute tenant.