I'm also struggling with the logic. I nearly bought a Y splitter when I bought the welder for £10, and I DID buy a flowmeter which was £10, so to add purge and another flowmeter I'm looking at £20 + whatever hose/fittings to get it to the job.
As for leaks, well you've got more potential for leaks adding another cylinder (because you've got another cylinder valve, another reg, another couple of gauge joints etc. - and you'd still want a flowmeter if you didn't want to throw gas away)