I don't object to investment in the existing rail network, as I still benefit as a longer distance traveller by rail, I don't object to investment in reopening of old lines/new lines and Light Rail, as long as the new line can meet its operating costs without subsidy, or there has to be an overwhelming economic benefit.
I agree any light rail scheme would have to be part of a wider scheme, and it gives more flexibility to diverge from the rail formation as appropriate. It could be that a light rail scheme didn't even use the Calder valley line but take a different route from low moor.
The main commuting destinations from Spen Valley are Leeds and Bradford, there is some demand for Dewsbury and Huddersfield as they are in the same administrative region, I don't see a great demand for Wakefield specifically and Bradford could be served directly to Wakefield via the Halifax route which it already is to a limited degree with GC, so a better end destination for the line would be Dewsbury in my view, alternatively heavy rail to Dewsbury and onto Leeds, which would require significant work to access the main Dewsbury Line and has been discussed at length previously..
From a practical viewpoint Heckmondwike station area is now a housing estate, there is room to get a line through but not much else, and I imagime you will have a housing estate full of nimbys for starters. Also unlike many so called rail enthusiasts who seem to think that every rail line should be reopened especially ones that have most of the track bed intact, I'm not sure there is really the demand from the local population at this point in time.