[detaro]

Why allow them if you think they probably are more trouble than they are worth? (no way to contact them if there are payment issues, easier to take-over accounts, ...)

IMHO it really depends on the kind of service if this makes sense or not.

[aw3c2]

Totally depends but in the majority of cases I see no issues. Why do you assume they are up to cause trouble? I used throw-away mails hundreds of times and the only thing I usually did was inflate their user table with another single time visitor.