Tokoro can be used in the abstract, for one thing.
"Shitsu" alone doesn't mean "room". You find it at the end of words which are the names of types of rooms. 待合室 (machiaishitsu) or 休憩室 (kyuukeishitsu), for example. When read alone, that kanji is "muro", a word not often heard. To speak of rooms in a general way, 部屋 (heya) is commonly used.