Many DBAs regularly use SYS -- either in SQLPlus or in Enterprise Manager or in RMAN scripts. DBAs not familiar with the reasoning behind the "AS SYSDBA" might not even be aware that they connect as SYS.
I prefer to avoid using SYS as far as possible. There is no reason to use SYS to query data dictionary views and performance views if you can use DBSNMP or PERFSTAT or setup an account with the MONITORER role. Best would be to setup an account with the required privileges, reading from the privileges granted to DBSNMP and PERFSTAT.
Besides the fact that SYS is owner of the data dictionary, the "normal rules" of behaviour for monitoring and DBA accounts do not always apply to SYS. For example SYS does not get audited in the normal database audit defined by AUDIT_TRAIL. 9i introduced AUDIT_SYS_OPERATIONS to cater to the requirement to audit SYS but this does not provide the same structure as the DBA_AUDIT% views. Another difference in SYS is in read consistency. If you use SYS for Exports, please stop doing so. SYS may be used for Transportable Tablespaces in 8i/9i or under Oracle Support's direction but *not* for regular Exports. If you have any doubts, please read MetaLink Note#277237.1.