It does make a difference, so it’s worth pursuing. Some examples:
possible access to disease modifying drugs: they aren’t a cure, but they do reduce the number of relapses that people have, the severity of the ones that they still get, and delay and reduce disability.
easier access to meds for symptoms: unless you have a really supportive GP, getting meds can be hard if you aren’t diagnosed
easier access to therapy & support generally: PCTs don’t generally like funding treatment for undiagnosed conditions and (although it’s wrong) medical professionals and associated professionals (e.g. occupational therapy) take you more seriously if you have a label
it’s easier to explain your symptoms to people!
you are protected by The Equality Act
you are more likely to get DLA / ESA if you have a diagnosis
There are bound to be others too. Ultimately, however, the best thing about having a diagnosis is knowing what’s wrong.
Of course, this assumes that someone really does have MS. Pursuing an MS diagnosis when it isn’t MS would be a terrible waste of time and energy, never mind miss the opportunity to potentially get cured for what is wrong!
Overall, in my opinion, a diagnosis is well worth fighting for. But I don’t know if I would feel that way if I’d been in limbo for years.