It depends on the person of course,but I think also in circumstance.

If I want to tinker or aimlessly play with something,sure,artificial deadlines may not suit me.

But if I want to even remotely accomplish an actual goal,or build something,or even learn something concrete,I positively need a firm and ideally external and enforced deadline - otherwise I'll meander,change scope and direction,redo from scratch,try a different tool / framework / system / method /whatever...and generally never ever finish