Customers often want to know why they aren’t supposed to update their data via raw SQL and are instead directed to use the CRM web forms or the SDK.

These are the reasons I am aware of:

1) Various important parts of CRM rely on C# code that will only run properly if everything is accessed through the web services (plugins and workflows)

2) Microsoft wants to be able to change the inner workings in future upgrades, and if everyone developed relying on particular columns being in particular tables upgrades would become impossible.

3) It's a complicated system and it's easy to miss a detail, such as the isdeleted column or state codes or timezone conversions.  These details can be abstracted away for read only access when querying against the filtered views, which automatically removed columns with the deleted flag and convert utc timezones to local timezone and check that the user querying has permission to view the record, etc.  But you can't readily do such abstraction for updating.

4) There is a security model in place that gives each user permissions to do certain things based on roles granted to them, and if you rely on working as a priviledged user in the database you might accidentally circumvent that. 

5) Some calculated fields like totals are 'not valid for update', but this is only enforced at the web services level (for example, totals can’t be directly updated because any value you might try to submit will be overwritten by a plugin calculating it).

6) There is an added level of complexity at the raw database level - base vs extensionbase tables, for example - that is more difficult to work with.

7) One wrong value can lead to system errors.  For example, deleting a row from the extensionbase but not the base table, or setting a picklist to an impossible value, can cause spurious errors when later trying to work with the information through the normal interface.

8) Schema changes in particular are tracked with various metadata that you really don't want to get into and you can break everything in very difficult to recover ways by manually changing column definitions.