And so the disciples asked Our Lord how to pray and he taught them... the Our Father.
So biblically we have 1 case of a rote prayer, from Our Lord's lips which is a good set prayer to say. Of course the words of the play Hamlet are also true "words go up, thoughts stay below, words without thought do not to heaven go".
Which is the threat to any prayer, less with spontaneous ones, but real nevertheless. The solution is not to make a dogma out of not saying standard prayers, but to pay attention to them!
I do this by changing the emphasis and timing, by pausing after lines so as to reflect on what I've just asked for.
Bless us Oh Lord [pause]
and these, thy gifts - [insert here spontaneous specified gifts or remain silent]
which we are about to receive from THY bounty, THROUGH Christ, Our Lord,
Amen.
In the name of the Father and of the Son and of the Holy Spirit, Amen.
Point 1) nothing in this set prayer is wrong, bad, awful per se.
Point 2) when otherwise distracted a skeleton of a set prayer helps give direction to one's stray thoughts.
When we sing Pslams we also recite someone else's prayers. It's great to ad lib and make things up as you go, I'm as much a poet as anyone so I appreciate the spontaneity. But like everything, balance is a support, not hindrance to growth.