I don't think it is necessary at all. For certain types of therapy, if one wants to use the relationship with the T as a model to explore attachment patterns and mechanisms, sure it can be useful. But, I think, it is perfectly possible to equally meaningfully discuss the client's other attachment and relationship style with other people. I personally think the latter can be more effective and productive as it would not be muddled by the unnatural relationship that therapy itself is. I personally can dissect and experiment with relationships and attachment patterns in everyday life much more effectively than in therapy as those occur in a more real context. I think once you have gotten to a certain level of self-awareness and are comfortable with facing your patterns, it is pretty easy (at least for me).

Regardless, many people seem to find exploring the therapy attachment useful especially with certain modalities.