Constantly querying system views [Oracle, MSSQL, PostgreSQL]

I'm a kind-of-experienced database developer that just got into the DBAdministration world. I've been told that in both Oracle and MSSQL is definitely a terrible idea to run queries such as the below example, **constantly** every, let's say, 5 seconds. Since it might cause memory fragmentation.

SELECT  *
FROM    V_$SQL_MONITOR; --oracle

I've done some research, but I couldn't find any official (or unofficial, for what is worth) documentation discouraging to do such a thing. Could someone experienced in the DBA world illustrate for me if this is indeed a bad idea, what memory fragmentation is, and if this practice also applies to PostgreSQL? Thank you in advance!