Is it necessary?
Dam, my Opemus III doesn't allows for it; luckily both the neg and the paper are not aware of it! (-:
Seriously, for the maximum light this is true, but I've always used 6x6 enlargers (a stolen Durst 600 and now the Opemus) for 35mm with the 6x6 condensers and today I print 8x10 in about 10s at f8 with a 75W bulb.
Doesn't need it. But should have it, for optimal results.
I use my Opemus 6 with colour head for everything small enough to fit in the carrier - 24x36 to 6x6 - with the same mixing chamber. No problems at all. I have more difficulties with 6x7 in my 13x18cm enlarger...
I have a 50, a 60 and an 80mm lens for the opemus. The 50 has been used once (needed a really big enlargement from a small part of a negative, the 60 never, and the 80 the rest of the time.
Just one more thing to remember. When you use a 50mm lens you must invert the lens mounting ring (it is slightly conical in shape) or you won't be able to focus properly. I used one of these enlargers with a Meograde VC head and was very impressed by its performance. These enlargers may not be the prettiest on the planet but they are tough and long lasting. The metal negative carrier is very well made. :)