I agree in principle. However, there are examples of "minor agreements" which put Q completely in question. Let me give an example.
| Mark 2 | Matt 12 | Luke 6 | Commentary |
23 Καὶ ἐγένετο αὐτὸν ἐν τοῖς σάββασιν παραπορεύεσθαι διὰ τῶν σπορίμων, καὶ οἱ μαθηταὶ αὐτοῦ ἤρξαντο ὁδὸν ποιεῖν τίλλοντες τοὺς στάχυας.
23 One Sabbath he was going through the grainfields, and as they made their way, his disciples began to pluck heads of grain | 1 Ἐν ἐκείνῳ τῷ καιρῷ ἐπορεύθη ὁ Ἰησοῦς τοῖς σάββασιν διὰ τῶν σπορίμων· οἱ δὲ μαθηταὶ αὐτοῦ ἐπείνασαν καὶ ἤρξαντο τίλλειν στάχυας καὶ ἐσθίειν.
1 At that time Jesus went through the grainfields on the Sabbath. His disciples were hungry, and they began to pluck heads of grain and to eat. | 1 Ἐγένετο δὲ ἐν σαββάτῳ διαπορεύεσθαι αὐτὸν διὰ σπορίμων, καὶ ἔτιλλον οἱ μαθηταὶ αὐτοῦ καὶ ἤσθιον τοὺς στάχυας ψώχοντες ταῖς χερσίν.
1 On a Sabbath, while he was going through the grainfields, his disciples plucked and ate some heads of grain, rubbing them in their hands. | Matt & Luke added the eating of the grain |
24 καὶ οἱ Φαρισαῖοι ἔλεγον αὐτῷ, Ἴδε τί ποιοῦσιν τοῖς σάββασιν ὃ οὐκ ἔξεστιν;
24 And the Pharisees were saying to him, “Look, why are they doing what is not lawful on the Sabbath?” | 2 οἱ δὲ Φαρισαῖοι ἰδόντες εἶπαν αὐτῷ, Ἰδοὺ οἱ μαθηταί σου ποιοῦσιν ὃ οὐκ ἔξεστιν ποιεῖν ἐν σαββάτῳ.
2 But when the Pharisees saw it, they said to him, “Look, your disciples are doing what is not lawful to do on the Sabbath.” | 2 τινὲς δὲ τῶν Φαρισαίων εἶπαν, Τί ποιεῖτε ὃ οὐκ ἔξεστιν τοῖς σάββασιν;
2 But some of the Pharisees said, “Why are you doing what is not lawful to do on the Sabbath?” | Matt & Luke used Mark´s “ἔλεγον“ in the form of „εἶπαν” |
25 καὶ λέγει αὐτοῖς, Οὐδέποτε ἀνέγνωτε τί ἐποίησεν Δαυὶδ ὅτε χρείαν ἔσχεν καὶ ἐπείνασεν αὐτὸς καὶ οἱ μετ᾽ αὐτοῦ,
25 And he said to them, “Have you never read what David did, when he was in need and was hungry, he and those who were with him: | 3 ὁ δὲ εἶπεν αὐτοῖς, Οὐκ ἀνέγνωτε τί ἐποίησεν Δαυὶδ ὅτε ἐπείνασεν καὶ οἱ μετ᾽ αὐτοῦ,
3 He said to them, “Have you not read what David did when he was hungry, and those who were with him: | 3 καὶ ἀποκριθεὶς πρὸς αὐτοὺς εἶπεν ὁ Ἰησοῦς, Οὐδὲ τοῦτο ἀνέγνωτε ὃ ἐποίησεν Δαυὶδ ὅτε ἐπείνασεν αὐτὸς καὶ οἱ μετ᾽ αὐτοῦ [ὄντες],
3 And Jesus answered them, “Have you not read what David did when he was hungry, he and those who were with him: | Matt & Luke skipped Mark´s “David was in need” and used Mark´s “λέγει“ in the form of „εἶπεν” |
26 πῶς εἰσῆλθεν εἰς τὸν οἶκον τοῦ θεοῦ ἐπὶ Ἀβιαθὰρ ἀρχιερέως καὶ τοὺς ἄρτους τῆς προθέσεως ἔφαγεν, οὓς οὐκ ἔξεστιν φαγεῖν εἰ μὴ τοὺς ἱερεῖς, καὶ ἔδωκεν καὶ τοῖς σὺν αὐτῷ οὖσιν;
26 how he entered the house of God, in the time of Abiathar the high priest, and ate the bread of the Presence, which it is not lawful for any but the priests to eat, and also gave it to those who were with him?” | 4 πῶς εἰσῆλθεν εἰς τὸν οἶκον τοῦ θεοῦ καὶ τοὺς ἄρτους τῆς προθέσεως ἔφαγον, ὃ οὐκ ἐξὸν ἦν αὐτῷ φαγεῖν οὐδὲ τοῖς μετ᾽ αὐτοῦ εἰ μὴ τοῖς ἱερεῦσιν μόνοις;
4 how he entered the house of God and ate the bread of the Presence, which it was not lawful for him to eat nor for those who were with him, but only for the priests? | 4 [ὡς] εἰσῆλθεν εἰς τὸν οἶκον τοῦ θεοῦ καὶ τοὺς ἄρτους τῆς προθέσεως λαβὼν ἔφαγεν καὶ ἔδωκεν τοῖς μετ᾽ αὐτοῦ, οὓς οὐκ ἔξεστιν φαγεῖν εἰ μὴ μόνους τοὺς ἱερεῖς;
4 how he entered the house of God and took and ate the bread of the Presence, which is not lawful for any but the priests to eat, and also gave it to those with him?” | Matt & Luke skipped Mark´s “Abiathar the high priest” and used “μόνοις/μόνους“ (only for the priests) |
| x | 5 ἢ οὐκ ἀνέγνωτε ἐν τῷ νόμῳ ὅτι τοῖς σάββασιν οἱ ἱερεῖς ἐν τῷ ἱερῷ τὸ σάββατον βεβηλοῦσιν καὶ ἀναίτιοί εἰσιν;
5 Or have you not read in the Law how on the Sabbath the priests in the temple profane the Sabbath and are guiltless? | x | x |
| x | 6 λέγω δὲ ὑμῖν ὅτι τοῦ ἱεροῦ μεῖζόν ἐστιν ὧδε.
6 I tell you, something greater than the temple is here. | x | x |
| x | 7 εἰ δὲ ἐγνώκειτε τί ἐστιν, Ἔλεος θέλω καὶ οὐ θυσίαν, οὐκ ἂν κατεδικάσατε τοὺς ἀναιτίους.
7 And if you had known what this means, ‘I desire mercy, and not sacrifice,’ you would not have condemned the guiltless. | x | x |
27 καὶ ἔλεγεν αὐτοῖς, Τὸ σάββατον διὰ τὸν ἄνθρωπον ἐγένετο καὶ οὐχ ὁ ἄνθρωπος διὰ τὸ σάββατον
27 And he said to them, “The Sabbath was made for man, not man for the Sabbath. | x | 5 καὶ ἔλεγεν αὐτοῖς,
5 And he said to them, | Matt & Luke skipped Mk 2:27, but Luke have “And he said to them,” like Mark |
28 ὥστε κύριός ἐστιν ὁ υἱὸς τοῦ ἀνθρώπου καὶ τοῦ σαββάτου.
28 So the Son of Man is lord even of the Sabbath.” | 8 κύριος γάρ ἐστιν τοῦ σαββάτου ὁ υἱὸς τοῦ ἀνθρώπου.
8 For the Son of Man is lord of the Sabbath.” | 5 καὶ ἔλεγεν αὐτοῖς, Κύριός ἐστιν τοῦ σαββάτου ὁ υἱὸς τοῦ ἀνθρώπου.
5 And he said to them, “The Son of Man is lord of the Sabbath.” | Matt & Luke used the same word order |
I think it is not possible that Matthew and Luke have so many agreements independently of each other. On the other hand, we also have clear differences between Matthew and Luke, which are good arguments against the assumption of a harmonization.