Yes it drops onto the wrong line really doesn't it? If it carried on over the ECML and hitched onto the TP mainline it would be much more useful as it would help segregate that traffic a bit better on the approach to Leeds as the only significant clash would be with platform 17 as it would be a straight shot into 15 and 16 from that viaduct. But getting it to the TP mainline involves a new bridge over the ECML and compulsory purchase of several business and you still need to recondition the structure which I'm sure would be a massive undertaking before trying to do anything extravagant like taking it somewhere useful.
Holbeck viaduct is one of those things that is very very alluring because it's right there and Leeds is congested. But I'm not sure that it actually has a future as a piece of railway infrastructure.
Edit: Thinking about it, I think historically it did connect to the line through Dewsbury rather than the ECML. Presumably when it finally fell into disuse they just cut it back to where it currently sits just short of the ECML?
Further edit: Clearly I should finish my Googling before posting
So yes historically it was connected to Dewsbury but it appears that in the 1960s it was changed so that it connected onto the ECML instead becoming the main route into Leeds for London trains. A situation which continued until the late 80s when it was deemed unsuitable for electrification and then fell into disuse.