It's probably a perceived marketing opportunity rather than a need. There's an enormous difference in the percentage of planes that are half series in the UK vs. the US. The resulting used plane price shows it.
Last year, I ordered a record 4 1/2, and it cost 5 quid more than a record 4. Here in the states, for a while, the going rate for a good condition sweetheart era 4 (not a perfect collector plane, but a good plane without pitting, breaks or spent parts) was about $45, and a 4 1/2 as clean was over twice that.
Everyone has a theory (one of them here is that 4s were a stock item and 4 1/2s special order). The only practical difference that I can think of is that if you fan camber out wider on a 4, it could make a slightly more perfect surface in terms of scalloping if you actually finish off the plane.
I think the extra weight is seen as an asset to a beginner or someone who uses a plane only for a small part of the work. I was enamored with the 4 1/2 when I had a power jointer and used it an and a power planer for everything. Also liked the stanley 8 a lot. Since going mostly to hand work, including dimensioning, i only have one 4 1/2 - a bedrock with what looks like a bite mark out of the sidewall, because I found it for pennies and don't think it'd sell well. I have two 4s and use them regularly. I don't actually know anyone here in the states who does a lot of hand dimensioning and uses any half sized planes, and most of the hand work with the larger planes is done with wood when suitable. The only non-suitable time I can think of is when you're doing solely edge jointing with a woody, it'll create lateral concavity if the plane doesn't seen face work mixed in.
As far as Pat Leach, he knows a whole lot about stanley's planes, but I think some of his personal preferences don't match the open market of users. Like his belief that the #112 is perhaps stanley's best ever tool. It becomes useless for solid work if you know how to use the cap iron, and even if you don't, using it on anything but very flat wood is torturous. Presumably, it gained popularity for large veneer work? Don't know, but there would be more 112s than there are 4s if the market would've agreed with patrick's opinion. He also feels that a 6 is a useless plane, and it's definitely far from that.