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The short answer is that we don't want to show too many numbers on the main MultiBit HD screen.
Long answer:
Most bitcoiners are pretty numerate but a surprising number of people
don't like numbers.
If you see someone, say in a cafe, with a spreadsheet or adding up numbers and ask them what they are doing they will answer 'oh, it's just work'. They never say: "oh, just having some fun with numbers Check this out !'.
If you do a side by side comparision of MultiBit Classic and HD, say
here, you can see that the number of visual elements (and especially numbers) is much reduced. This is to keep things simple and reduce the cognitive load of using MultiBit HD.
Longer answer:
There are a few things in bitcoin which aren't as simple as they appear.
For instance: as Bitcoin is fully decentralised
time is a bit amorphous. Time in common parlance is always monotonic, you have a well defined 'before' and 'after' and hence can order transactions. You cannot
really do this in Bitcoin but all wallets fudge it slightly as we are so used to monotonically increasing time. (If you have done any special relativity it is similar, 'before' and 'after' are not well defined for
space-like intervals.
Also the idea of a
balance of a wallet at a particular point in time is a little bit conditional. A wallet is really saying:
"As far as I know from the information from the peers I am connected to, given the current state of the main chain, which may be reorganised at any time, the total of all the unspent transaction outputs under control of the private keys in this wallet is X BTC". There are a few ifs and buts in there.
You can quite legitimately have two versions of MultiBit HD running on different machines and they will see transactions at different times and in a different order and hence have different balances at a particular point in time.
With MultiBit HD we try to hide these complexities where possible and where it is safe to do so.