Field Exits - Aren't these no longer supported by SAP?

I have a request that appears it could be resolved by use of a field exit.

But aren't these no longer supported by SAP?

I thought SAP discouraged the use of them and would no longer support them from version 4.0 +. Wouldn't that mean you would have to reinstall them after an upgrade (and the some service packs) - since SAP would overlay them.

I seem to remember that because they work at the data element level that they can cause havoc when the data element is used in more than one table.

Can some one confirm this or shed some light on the topic?

Thanks.

Scott