I am not a programmer, so this issue doesn’t impact me on that front. The reality is that encountering a name with substitutes for spaces offends all my esthetic sensibilities; it just looks hideous, especially the underscores. Usability also goes down the drain with anything that requires such frequent use of the shift key.
Probably this would be a non-issue except for stone-age, intolerant Unix. We can also blame Microsoft, Apple and all the rest of the computing world for implementing long filenames without carefully considering all the implications. This is part of the ongoing campaign to make computers “user friendly” so they can be operated by complete novices, but it has resulted in a giant tower of Babel.
As happens each time everybody sits in their corner and does their own thing, we now have a big mess which will probably never be resolved.
I agree with the above comment that computers are supposed to work for us; we shouldn’t be required to adapt our lifestyle to suit them.
In this case, the number of space-intolerant scenarios is relatively small, so it is illogical that we all have to modify our behaviour to cater for them.
In situations like this, a de facto standard usually emerges and saves the day. Since none of the conventions being used today is acceptable, this unfortunately doesn’t look like happening any time soon.