It makes no sense to an end user that expects an argument you pass to BigFloat to be treated as a BigFloat. As an end-user I would rather have a warning or even error than to have my argument silently treated as a double.
BigFloat(x::AbstractString) is identical to parse. This is provided for convenience since decimal literals are converted to
Float64 when parsed, so BigFloat(2.1) may not yield what you expect.
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Examples
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julia> BigFloat(2.1) # 2.1 here is a Float64
2.100000000000000088817841970012523233890533447265625
The solution here is a good linter though, not adding more work to the already overstressed compiler. It comes back to the issue of Julia needing more mature, easy-to-work-with tooling, that could say "hey, this is technically allowed, but you probably didn't mean this".
And this is not an advantage to the designer exclusively, it is very much an advantage for the end user that the treatment is explicit, consistent and predictable, instead of 'magically' reinterpreting the meaning of literals based on guessing the intent of the user.
Basically, you seem to be saying that when passing x to BigFloat, x should not be treated as the value x, but as some nearby value that might be the one the caller intended (based on some rounding logic perhaps?) Or are you perhaps saying that
x = 1e-300
y = BigFloat(x)
y = BigFloat(1e-300)
I think it was a mistake. Invariably, a new user discovers the discrepancy and is thoroughly confused. Let's not repeat the same mistake with big nums.