Depends on the situation as to whether it's detrimental. Proxy-aggregation,
as this is called, can alter traffic patterns in dual-homed situations. The
change is not always bad, though it is often un-desirable.
In general, proxy-aggregation is good for everybody.
If we had a good method for people to indicate routes that they didn't want to
be aggregated, then more proxy aggregation could be done safely.