It's a matter of weight and the amount of use it will get. Two adults sleeping in a mattress every night have different support and comfort requirements than an occasional guest.

With a guest bed, the question is: "How long do you want your guest to stay?" Older children may be upfront by complaining about the old set in the guestroom but your important non-family guests will either openly compliment you on your choice of the mattress in the guestroom or quietly complain to themselves and cut their visit short, making an unanticipated excuse as to why they have to leave.

If you are considering replacing your guest bedroom mattress with a cheaper, low quality model, consider putting your (adequate) current set in the guest room and choosing a new set yourselves.