Put on a set of decent semi-fasts Paddington to Reading and dedicate the Intercity services to people making longer journeys, rather than them carrying fresh air further west.
In what paths? That's the problem there's no more paths available.
Even if you could do that, you're still looking at there not being 1/2 full trains, so if demand doubled (which it could easily do over the next 20 to 30 years, whilst Crossrail 2 is likely to be 15 years away).
Even with the advent of the Exeter semi fast, all that'll do is increase demand faster. It also doesn't take into account the doubling of demand on the WofE as well. Which would likely justify 2tph to Exeter as soon as it's possible (most of the time I've used the services they are already fairly well used along most of not all of their length).
Therefore creating a third service an hour probably would have enough demand to justify its existence in the same sort of timeframe (circa 30 years).
If you are running those sorts of frequencies (4 or 5 tph to Exeter from London/South) then you're going to want to be considering extra services to Plymouth as well.
The existing line won't have the capacity for more fast services, the DAL would be facing the wrong way for the WofE line services (as well as being very expensive for not a lot of extra gain).
The Okehampton route is cheaper, allows an easy extension of the WofE line services, speeds up journey times from the South (places like Brighton, Woking, Southampton, Weymouth) to Plymouth, connects Okehampton to the rail network and provides Tavistock with a better link to places other than Plymouth & Cornwall.
It's it needed right now? No other than as an insurance policy for the main line.
Does that mean that it wouldn't be useful in the future? No, and so it should be filed as a hero an eye on what's happening project, so that when there's extra paths available there's a plan as to how they could be used. Especially to bring benefit from what would otherwise be a fairly London centric project (Crossrail 2) to the wider country (well at least the South West).