The SS piston is mainly for people that do not feel comfortable using "plastic" parts in the high pressure/high temperature chamber. Although the material we use is FDA food and drink contact approved and it's BPA free, and of course withstand temps much higher than boiling, some people just don't thrust that there are special "plastic" blends/grades that are totally safe.
The second reason is that once you heat up the SS piston it will remain hot for a long time, and the temperature will not move at all during the brewing. In practical terms, it is very difficult to identify this difference in the taste, aroma, or appearance of the espresso.
Yes, aesthetics/value is the 3rd reason.
If those are not important to you, you can stay safely with the regular piston.