It depends, but most likely, if you're only MNK and want to be useful, then yes. Your problem is with such a lack of diversity or in the case of a single job, no diversity at all, you cannot change jobs to fit the situation. In the case where feeding TP becomes increasingly bad or you just cannot do decent physical damage, Chi-Blast is the only reasonable option left open to you. However, without a diverse range of jobs, you really preclude yourself from several activities and can even hurt your chances at end game activities especially when the sole job you chose lacks an pivotal role.