I suppose yet again it's "caveat emptor" the auctioneers who are charging larger fees obviously have a market for those fees otherwise they wouldn't be in business. It's up to the vendor (or purchaser of auctioneering services) to check out which auctioneer would suit their needs the best. In a way they are giving you their opinion of the value of their services
With regards to the auctioneers closing sales quickly and not chasing the possible maximum values on a property:
Small fees mean tight margins - this means that the more time they dedicate to getting in offers, the less cost effective it is for them as there are only so many hours in the day. In order for the auctioneer to maximise their profit margin, they would need to get in as many properties as possible with a swift turnaround.
IMHO this is certainly not best practice as I would engage an auctioneer to look after my best interests first and his second. As the employer in this relationship I would be wary of engaging someone who would operate like this.