I do not quite agree.

It's such a generalisation and such a loose assumption that once you go offensive you would be making mistakes.
One can make mistakes anytime and anyhow.
They do not need to be on the offensive and defensive to make mistakes.
Once they fail to make proper assessments (ie being foolish) of their decisions, mistakes tend to happen and losses will be made.

Use real examples.
Reits are defensive right?
I will use IGBReit price of 1.40 (and not 1.45 - which is the highest) as an example.
At 1.40 and wiith a projected/promised dividend of 6.3 sen, the projected yield of the buyer is 4.5%.
How many warned and reasoned that being defensive by buying IGBReit is a risky/bad investing/trading decision?
IGBReit hit low of 1.25 before recovering to 1.3.
These 1.40 buyers would be collecting their 3.43 sen but their defensive decision of buying at 1.40 is losing them money at the moment.

Offensive.
Best recent and live example here is Superman.
Superman can be considered an offensive buying, right?
My suggested 'initial' (I think I could have entered at several other prices - even entering at 2.20. ) was 2.00 plus or minus 2 sen.
Price yesterday was 2.38.
If I had purchased I would still be on the offensive and be holding the stock.
I still see no reason to sell the stock for now.

I too believe in locking in the profits and to a certain extent, one must know how to take losses if one is wrong. (Of course ideally and more importantly, you need to try to avoid being wrong when you buy)
But I believe more in trading/investing correctly.
Profits and losses should not be simply taken.
If you trade/invest correctly, profits would reward you handsomely.

If profits are taken just for the sake of locking in profits, one will tend to trade/invest excessively and more often that not, this will yield an unsatisfactory results.

My few sen worth.