Being useless in the Summer didn't stop the UK deploying both the Lynx 7 and Merlin to Afg; why? For the former it was a cynical AAC push to protect Wildcat Funding, for the latter an attempt to spin out of bad headlines re helicopter numbers in Theatre.
The Merlin is an "or" helicopter; it will carry 20 troops, or fly for 4 hours or lift a 3000kg USL. The CH47 can do all three, at the same time. So what for a maritime helicopter you say? Well, having operated CH47s off one of our, ahem, pocket-carriers for an extended period (and a grad of the RN Amphib Ops Cse) it's actually massively important. It's all about deck cycles and spot numbers - if a CH47 takes up an extra half spot to a Me but carries gusting 4 times the payload it's a massive force multiplier. Yes you can launch wave 1, lift, unfold then launch wave 2 but remember your wave 1 is carrying less payload and more gas (in an 'or' helicopter.....) to make this possible.
As for 'losing' by keeping Puma vice Merlin...I'd have a very close look at the high DA perf figures before saying that; that's before you add a few hundred kilos if folding kit back into it without a gearbox upgrade. Despite what people will say, CHF have flown the SK in several high DA theatres over the past few years (Bosnia in Summer, Iraq and Afg) and will doubtless do so again when called. To put it simply, CHF will embark for exercises and photo shoots - if a real Op comes up one of the first things that will happen will be 5 CH47s arriving to boost the CAG with real heavy lift.