Actually, your LHS is dead on correct. And Most
decent modern chargers actually have a HV setting. Or able to adjust the Voltage cut-off to 4.35v per cell.
But yeah you can just charge it at 4.2v, and you will be fine.

Yeah the cells will last longer because you are never peaking out the cells to their max. designed voltage. And perfectly ok to do so , any Lipo can be charged below its peak voltage and
this makes them last slightly longer, at least in theory. Just that HV packs are much more $expensive$.
Is it really worth it in the end?
Only you and your wallet can answer that. SMC makes good packs in general. Just that HV packs are not commonly purchased. Not much of a thing. Been around for some years now. I think the ESC mfrs, haven't quite embraced HV packs completely, yet. I see many LHS's/vendors being "stuck" with them. Nothing wrong with them. Except just being more expensive.
And...
the most important thing is that The ESC must be able to handle a HV pack that IS charged at 4.35v per cell., if you intend to.
Most decent ESC's that
cannot run HV Pack's voltage, will see an Over-voltage at start up, and Not intialize and Power
On. (A fail safe) Best case scenario.
Worse case the ESC becomes toast in a split second.
Which Charger are you currently using now? Doesn't make sense to buy one that is HV settable, if your ESC won't handle it. And a regular Non HV charger will charge an HV pack to the max of 4.2v per cell anyway. If that is your intent. Why spend
all the extra coin on a HV charger and HV pack if you wont use the 4.35v anyway? Not practical. I am sure your LHS will want to sell you both.

EDITED a few times for clarity.