Is it really best for kids to keep in touch with absent parent?

[evilstepmom]

my ss (6) and sd (3)see their mother every other weekend from saturday afternoon until sunday afternoon. wkend just gone they stayed from fri nite til sun afternoon. they used to do this as standard but stopped in new year as they came home grumpy, tired, dirty, sulky,whiny etc etc. just staying one night still come home all of those things but not quite as bad.
so far this week we have had children from hell. to cut long story short, if they are nice normal well adjusted children the majority of the time why send them somewhere to just seemingly mess them up!!
if they dont see their mother for a while (which frequently happens as she cancels a lot)they dont even question it and are the lovely kids we know and love.
it just seems like we end up with a miserable household every time they go.
is it really fair and "good" for the kids to keep in contact with someone who really does not seem to be a positive influence in their lives?!
sorry for rant, this has been going on for nearly three years with no respite...

mmm, I lost contact with my dad around age of 5, was very upset at the time but soon forgot him because he didnt flip in and out of my life. It wasnt till i was 16 that i thought about contacting him and found that i didnt feel very much for him then. So was better for me to of been left alone. My mum was good in as much as she never bad mouthed him so I had a pretty view of the situation may be that helped. too much is said in front of the children today.

thank you everyone for all your advice and comments - really helpful to have so many perspectives on the matter. TCOTB - I totally see where you are coming from and in the case of dss it may well be because he is missing her (dsd tends to follow his lead). but he never asks about her and seems disinterested if she ever phones him. poor little chap, who knows what he thinks really. anniemac - we have offered her alternative and extra contact arrangements (ie taking them out one night in the week) but she always refuses and says she is too busy.
dh and i have a rule that we (him and I) do not talk about her in front of/in earshot of the kids - obviously if they ask a question or mention her then we answer or acknowledge as positively as we can but in general we go with "if you can't say anything nice don't say anything at all!" Kids are welcome to talk about her as much as they like which we have tried to make clear to them.Think they are ok with that.
i guess you wise folk are right - only time will tell and hopefully the skids will grow up knowing that we tried to do what we thought was right for them at the time. if they end up with a positive relationship with their mother, then good for them and her but if not, then i hope that we can be there for them and support them and not be blamed, as although it was my question that started this thread, i think (at the moment!)that we will just have to persevere til they are old enough to make up their own minds. best of luck and thank you to all you "lovelystepmoms" out there!:)