Holmes reasons for not accompanying Watson are false. Holmes does not accompany Dr. Watson and Baskerville to the hall because he plans to come disguised to the area. He wants to investigate alone hoping to discover the secret of the hound and its apparition. Disguised as a beggar, he not only discovers the possible hiding place of the dog, but also the hiding place of Seldon the escaped convict and his relationship to the servants at the hall. He implies that he will be available only if worse comes to worse, but he is, in fact, present during the entire time. He quickly eliminates the red herring of Seldon and the hall's housekeeper and butler and discovers other possible suspects lurking about.
This is not an unusual situation for Holmes. In many of his stories he has "hiding" places all over London where he quickly disguises himself in order to follow or observe situations and people. Some times he is so well disguised that even Watson does not recognize him. It is one more tool in his arsenal of weapons to prevent or solve crime.
In Ch. 5 Holmes advises Sir Henry Baskerville not to go to Dartmoor alone. Henry replies that Dr.Mortimer is accompanying him to Dartmoor. Holmes tells him that Dr.Mortimer "with all the good will in the world" will not be able to help him because he will be busy with his practice and that he stays far from his house. At once, Henry asks him whether Holmes himself will come with him to Dartmoor. Holmes replies, "if matters came to a crisis I should endeavour to be present in person" but right now it would "be impossible for me to be absent from London for an indefinite time."
Holmes tells Sir Henry that he is very busy at the moment with people constantly calling on him to help them solve their problems. He tells him that even now he is busily engaged in saving the reputation of a very prominent person in London society from being scandalised by a wicked blackmailer: "At the present instant one of the most revered names in England is being besmirched by a blackmailer, and only I can stop a disastrous scandal."