you need to employ maths and each persons circumstances are different.
For example if a handset was £500 sim free but you could get it free for £20 a month for 2 years with some free calls and texts then technically you've got a decent deal because you've paid less than the full price and got some stuff free.
The other way is that same phone is £35 a month for 2 years and "free" so it ultimately costs you £760 for 2 years although presumably you'd get more "free" calls and texts.
The final solution is to buy the handset sim free and then get a sim only or pay as you go deal which is normally much less restrictive, no long term tie ins and generally more generous with the "free" things like calls and texts but then you have ti stump up the £500 (or whatever the charge is) which many people just can't do.
So in out and out financial terms it does often work out beneficial to go the sim free route but only if you're in a position to drop several hundred pounds into a handset which many aren't regardless of how it stacks up as finalised numbers.
Bit like it would cost you a million quid or whatever to buy a £300k house over 30 years but most people don't have the £300k to start off with.
Its horses for courses, no right or wrong although there's often a fairly polarised debate on it.
I've gone sim free a few times because I like changing a lot, I've never had a handset for the whole contract term I originally signed up to I don't think and thats over nearly 20 years of ownership. Thats a different debate though, I know I pay a small premium for the freedom to chop and change.