I was answering a CRM forum question on unsupported changes in CRM. The developer wasn’t really sure why it was important not to put in unsupported changes into CRM.
Here are my thoughts on why you should not put in unsupported customizations
The main problem with unsupported changes in Microsoft Dynamics CRM, is there is no guarantee they will work in the future because it may use/reference some functionality that Microsoft will change in the future or possible remove
if your CRM instance has a problem and there are unsupported changes in the code Microsoft support will not help you (only offer limited support) because the unsupported changes may be the cause of the problems you are experiencing.
finally when it comes to upgrading the solution, the unsupported changes may not work and could cause difficulties when upgrading. This probably seems a long way off when you are putting the change in but upgrading versions of CRM will involve a lot of checking anyway, you don’t want to make more work for yourself later.
It can be difficult because sometimes the customer really wants the unsupported customization but you need to inform them of the drawbacks of doing this
MVP writes about the dangers here
Below are some links to what are unsupported and supported changes in the various different versions of CRM
Unsupported changes in CRM 2013
supported and unsupported changes in CRM 2011
CRM 4 – unsupported customizations