Here's a summary of what's going on

1. Cashtab stores token balances in wallet.state as a BigNumber, at wallet.state.tokens[].balance
2. When wallet is saved to indexedDb using localForage, the BigNumber type is not preserved
3. So, when wallet is loaded from localStorage, you have something that looks like

wallet.state.tokens[0].balance = {
    "s": 1,
    "e": 0,
    "c": [
        1
    ]
}

We aren't checking whether or not this is a BigNumber (BigNumber.isBigNumber(x)) -- we are converting this to a BigNumber, since we know that's what it is supposed to be. Unfortunately, new BigNumber(wallet.state.tokens[0].balance) (the above value) does not work unless thisTokenBalance._isBigNumber = true is added first.

To your point, it should be removed -- but the way to do this is to make sure wallet.state.tokens[].balance is always stored as a string and not as a BigNumber, and always convert to BigNumber when you are doing calculations.

There's a longstanding task for getting this fix in. Since we are looking at some wallet upgrades, now is probably the right time to do it and I'll add it to my workflow: T2614

However, I think it's worth landing this diff first since it patches a bug. There are some other wallet upgrades in the pipeline that will make this function obsolete or drastically different, but it's still worth getting the bug fixed now.