Post a pic of pro cycling and instantly validate your point. I like it.
Someone find me a pic of a pro on a triple crankset.

Not just any pro. A pro that nobody would accuse of sucking at climbing.
I'm just saying that you don't necessarily suck at climbing because you use a smaller gear than 39/28.

FWIW; I pretty much need a 28 for a five hour training ride in the mountains but hate the 11-28 on fast group rides or races. Gaps are too big, often find myself between gears. 11-25 tends to work out better in those situations.

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2013 Wilier Cento1 SR || 2009 Ridley Crossbow || 2011 Yeti AS-R 5 Carbon

I think a couple of people have mentioned the unwelcome wider gaps of the 11-28. Every choice has a compromise but doesn't the 11-28 represent a small one. How often does one get into a larger than 21 cog unless climbing something pretty nasty, when a 28 would come in handy.

Terrain around here is hilly and during a fast group ride, I like to keep it in the big ring. 11-25 works better for me in those situations.

You need to experiment and find what works best for you. No right answer for everyone.

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2013 Wilier Cento1 SR || 2009 Ridley Crossbow || 2011 Yeti AS-R 5 Carbon

I get you. In the big ring it would make a difference. I wasn't thinking. I like to use the big ring with the large cogs as well. Of course technically you can find the same gear without the crossover but sometimes it's easier to find the chosen gear and stay in the big ring. It feels different too.

I'm not accusing any pros of sucking at climbing. If alp d'huez is your weekly climb then by all means gear the hell out of it. Otherwise I maintain my stance, if you need more than 34x28 for your weekly climb, you suck at climbing. Addendum: if you need more than 39x28, you should be riding a compact crankset.

Is there anything to stop you doing a mix and match here?

By that I mean, I've just bought a 11-28 to replace my worn 11-25 (both Ultegra 6700) as I have some big sportives coming up in the Summer. I don't really use the 23 and 25 that much in normal riding so would assume the 3 joined cogs (21,23,25) have not worn as much as the bulk of the block on the old cassette.

Assuming that is the case and these 3 cogs mesh well with the new chain and new cassette parts, is there anything to stop me using that cluster of 3 with the rest of the new Ultegra cassette and just switching in the 21,24,28 as and when needed or will there be problems with wear rates across the cogs and chain that would introduce skipping if the 21,24,28 weren't used all that often (new cogs, old chain say)?

That's an interesting idea, matty. I think you can just check alignment of 21t cog and splines between the two clusters and it is the same, you should be safe mixing and matching them. However, if it is not, you'll likely have issues with shifting.