Arrangements with kids do change though and just because it began as midweek contact...doesn't mean it will be that way forever.
As kids get older, they have extra curricular activities and midweek contact may no longer work. Their mums may have a change in working pattern/started a new job after being a SAHM, which requires some change in the existing arrangements.
As they get into their teens...they probably won't want every weekend, because they miss out on seeing their friends and socialising.
I think there's a bit if naivity, if one expects things to remain the same for ever more.
If your DP has children things can always change, which would could mean the DC are with you more or even move in with you full time.
His Ex could become ill which results in the kids being with you more. She could pass away. These are possibilities.
To be honest...and I know opinions will differ, I don't think it's best to have a relationship with a guy who has kids if you're young. At least not a serious relationship where you live together.
I think you can afford to be picky in your choice of men at that age and save yourself the hassle of stepkids and Ex wife drama to deal with.
In my late teens and twenties, I turned down men with kids for those very reasons.
I'd be very disappointed if my DD (now 18), got involved with a man with kids...for the very reasons you've all mentioned.
If she was late 30s, then I could accept it, but with youth on your side you don't need to deal with this baggage.