In my opinion that's pretty tight of the landlord, but... then you could think about it another way.
Think about this scenario:
You own a house and live in it, but you're a bit strapped for cash and like the house. You let a few people live with you in the spare bedrooms and pay rent.
Then, the three people you live with and that pay the rent, all bring their partners over 3 times a week. You then live with 6 people.
Where do you draw the line?
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Although, obviously that's not the case, it's difficult,.. but if someone is bringing one person over,.. and staying in the same room, it doens't cost the landlord... so long as you don't turn up and make a lot of noise and irritate him/her. I mean, i'd have issues if it was regular and every time the person was blarring out music... but, if you keep yourself to yourself, are tidy.. and generally don't use all the hot water / use too much electricity / block the loo < lol, I can't see why it should be a problem!