On the surface it seems that there could be nothing wrong with wanting to be an ally to a social movement. Being an ally implies wanting to help, and what could be wrong with that?
There is definitely nothing wrong with seeing a problem in the world and wanting to change it. However the language of ‘wanting to help’ implies that it is not your problem. This implication is exacerbated by the way that the language of ally is used: white people can be allies to people of colour in anti-racism work; straight people can be allies to queer people in the fight for equal rights; able-bodied people can be allies to disabled people in making a more accessible world; and men can be allies to women in fighting sexism. The problem with this language is that it implies that these issues are problems for people of colour, queer people, disabled people and women, but that white people, straight people, able-bodied people and men can help. Not only does this miss the ways that white people, straight people, able-bodied people and men are part of creating or reproducing these problems (through denial of racism, accumulation of capital based on racist policies over generations, socially sanctioned heterosexual relationships and higher wages, just to name a few ways), but it also misses the ways that white people, straight people, able-bodied people and men might benefit from a changed culture (one that is, for example less segregated and more accessible).
This is not to say that everyone will benefit equally from a changed society. Or that creating and adjusting to changed social relationships will be easy. But I firmly believe that in order to create new ways of being with ourselves and with others, we need to get everyone involved in the process of creation.