The thing with Rifle men and Moterstrelki is you can two vet (22 points) them giving them identical rifle accuracy to FAMAS and better LAW/RPG accuracy. The M113 and BTR60 will have a much more accurate stunning HMG as well.
Much better cost effectiveness but slower and shorter ranged transport .
The Riflemen by themselves are the worst 10 point NATO infantry. But the fact the basic M113 costs only 5 makes them a quite decent infantry ; you can for example pay 18 deployment points to get vet 1 Riflemen (and a vet 1 M113). At vet 1, they have 7.5 and 6.5 accuracy for the M16 and M72 LAW respectively.
What makes you think that vetting Riflemen is more cost-effective than having more Riflemen?
It's the same debate as whether to vet units or not ; as we all know, when the accuracy of a unit is below 6, for a given budget, you get more firepower at maximum range by vetting your units than by buying green ones. Vetted units also means they're less susceptible to combat stress, and because they're less numerous for an equal budget, have a smaller logistical footprint.
However, there are some downsides ; as they're less numerous, they can cover less area and don't benefit as much from the morale bonus of the numbers.
But quantity isn't something I'd rely upon that much when using infantry in a slow APC ; for defensive purposes, 4 Riflemen squads are generally enough to very well defend a sector IMO, so there's no point in deploying more units.
For me, the compromise between veterancy and numbers (unless I have OVER 9000 deployment points) is to deploy Riflemen at vet 1 or 2. I prefer vet 1 because you pay them for 18 deployment points instead of a true cost of 18.75
, and because it's still cheaper than most infantry squads (Don't tell me you love deploying Fusiliers in the FV603 Saracen...).
It's also more interesting to deploy units that aren't vet 4, because the units that you deploy vetted are generally cheap in comparison to their targets, meaning they can vet pretty quickly for free.
For the question of compromise between quantity and veterancy, you can take a look at Tigga's replays IIRC ; he always deployed Marder VTS1s in groups of 3 vet 1 units. If he only cared about maximum firepower, he'd have used vet 4 VTS1s...