It's unlikely that there will ever be agreement on this one.

One school of thought argues that it is unprofessional for a club to feature floor singers and/or budding performers(s) as a support. After all, people have paid good money to see the guest artist or band.

On the other hand, many argue that folk clubs are failing in their duty if they don't encourage club members and local singers, In some venues, they are even seen as more important.

Edinburgh Folk Club usually has a pre-arranged opening spot and we still welcome floor singers albeit, due to time restrictions, it's always wise to ask in advance before the night starts. There's an impression in some quarters that we don't actively encourage the latter and this may be true but it's usually because there's "always someone there".
Our friends at Leith don't have floor spots at all but do have an opening spot where local acts are encouraged to apply while in many of the rural clubs there are more singers nights than anything else.

Personally, I'd rather things were more black and white.... i.e. either a guest night or a "floor singer, open mic, or whatever night". It's the hybrid scenario which always seems to cause the most contention.
(These are just my own views though)

However, in Edinburgh we are spoiled for choice and there are all manner of sessions, singarounds etc some of which are more organised than others. So, it's possible for the likes of ourselves and Leith to concentrate more on a concert format. Besides, the punters would be unlikely to come along to these clubs just to hear local singers etc when they can just as easily go to the pub.

I do realise that in rural areas and small towns circumstances are a little different. So, I appreciate there's more of a need to cater for the Senga McGlumphers of this world who want to get up there and "do a turn".